Remove `Pds:Icon` as dependency for Structure

Description

The intent of this PR is to remove all the usages of Pds::Icon within the repo. Once this PR is merged a new version of Structure will be released, adopted in Cloud UI (with the needed changes/fixes that are required by the update), and only at that point we will proceed with the complete removal and deprecation of the Pds::Icon component itself from the repo (this will be done in a follow-up PR, obviusly).

Changes

In this PR I have:

added the @hashicorp/ember-flight-icons package as dependency (so we can use the <FlightIcon> component)
- to be able to view the icons in Storybook, we need to injected the Flight icons sprite in the Storybook page's <head> element; this has been done using the previewHead function
- notice: I had to add @hashicorp/flight-icons as dev-dependency otherwise the linter would complain that in .main.js the require() would be “extraneous”
updated the markdown with instructions for the “install” of the Pds::Ember package
removed sample icons from main tests/dummy app template (that page us useless)
Pds::Avatar component → replaced Pds::Icon with FlightIcon
- ⚠️ Notice: as agreed with @heatherlarsen we have replaced the square "user" icon with a generic "user" icon without a border around it; this is probably OK because in HCP every user has a gravatar image (see thread)
Pds::ErrorMessage component → replaced Pds::Icon with FlightIcon
Pds::Select component → replaced Pds::Icon with FlightIcon in the component “facade”
Pds::Dropdown component → replaced Pds::Icon with FlightIcon in the "trigger" sub-component
Pds::Button component → replaced Pds::Icon with FlightIcon
Pds::GlobalHeader component → updated the docs/stories to use FlightIcon instead of Pds::Icon
Pds::CheckboxField component → replaced Pds::Icon with embedded SVG in the component “facade”
- in this specific case I didn’t use the FlightIcon component because the corresponding icon check would touch the bounding box of the container, and would require some heavy refactoring; so I’ve decided that it was easier/better to directly embed the SVG (there are just a few instances of Pds::CheckboxField in Cloud UI, and by the time this PR will land they will likely have already been replaced with the equivalent HDS component)
Tooltip modifier → updated the docs/stories to use FlightIcon instead of Pds::Icon
Toolbar component → updated the docs/stories to use FlightIcon instead of Pds::Icon
CtaLink component → updated the docs/stories to use FlightIcon instead of Pds::Icon
Link "component" → updated the docs/stories to use FlightIcon instead of Pds::Icon (this is not technically a component, it's just styling via CSS)

Testing

I have symlinked my local directory package/pds-ember to the cloud-ui/node_modules/@hashicorp/pds-ember folder in Cloud UI, run the application and tested that everything worked as expected.

Relevant links

Epic in Jira: https://hashicorp.atlassian.net/browse/HDS-477

🚧 TODOs

Once this PR is merged and the new Structure version is adopted in Cloud UI:

Pds::Button
- [x] check all the instances that have a @iconStart and/or @iconEnd argument, and replace the name of the icon according to the pre-defined mapping (but consider there may be exceptions, so there may be a need to check visually the outcome in some cases, and validate with product designers).
- I have run a codemod (see below) on the entire Cloud UI codebase to check for such instances but found zero occurrences of button with @iconStart/iconEnd arguments
Pds::Dropdown
- [x] make sure that if there are instances in which the Pds::Dropdown::Trigger has a custom @icon argument, the name of icon is updated to use the Flight icon instead of the Structure one (you can search for /.Trigger/, case sensitive).
  - the only occurrence I've found is in this file packages/cloud-ui-core/addon/components/more-menu/index.hbs but the Structure icon names used in the Trigger are more-horizontal and chevron-down which are the same in Flight, so no need to update the code, it just works.

Codemod used to check <Pds::Button> instances with @iconStart/@iconEnd arguments:

module.exports = function ({ source, path }, { parse, visit }) {
  const ast = parse(source);

  return visit(ast, (env) => {
    let { builders } = env.syntax;

    return {
      ElementNode(node) {
        if (node.tag.toLowerCase() === 'pds::button') {
          const hasIconArg = node.attributes.find(
            (a) => a.name === '@iconStart' || a.name === '@iconEnd'
          );
          if (hasIconArg) {
            console.info(
              'Found <Pds::Button> with @iconStart/@iconEnd argument',
              `\nFILE: "${path}"`
            );
          }
        }
      },
    };
  });
};

module.exports.type = 'hbs';

hashicorp / structure