update relative paths URLs using postcss-url

[matthewdunsdon]

The issue is due to the import of stylesheets not getting their URL paths rebased, which causes issues when referencing from CSS in a different directory.

src/styles/icons.css

@import "./icons/icon-pack-24.css";
@import "./icons/icon-pack-32.css";
/* ... */

src/styles/icons/icon-pack-24.css

.icon-badge-24 {
    background-image: url("../../assets/icons/badge-24px.png");
}
/* ... */

Image icon is at path <project-root>/src/assets/icons/badge-24px.png and when build with ez-build it gets coppied to <project-root>/lib/assets/icons/badge-24px.png

Description

Added a build step so that for relative URLs, the correct relative path chosen.

Motivation and Context

Motivations for this change are discussed in #48.

How Was This Tested?

Tests added and manual testing on a project.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My change follows the style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have read the contribution guidelines
• [x] I have added tests to cover my changes
• [ ] All new and existing tests passed

[codecov-io]

Codecov Report

Merging #53 into master will increase coverage by 1.65%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   93.37%   95.03%   +1.65%     
==========================================
  Files          15       15              
  Lines         317      322       +5     
==========================================
+ Hits          296      306      +10     
+ Misses         21       16       -5

Impacted Files	Coverage Δ
src/postbuild/rebase-prod-css.js	100% <100%> (+83.33%)	:arrow_up:
src/builder/css.js	100% <100%> (ø)	:arrow_up:

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[matthewdunsdon]

As this is a breaking change, we need to be strategic with how we merge and deploy these changes.

@mstade Can you make the call when the code gets merged in and deployed?