fix: add property to toggle switch for accessible desciption

Description

Added a new property, description to the <md-toggle-switch /> web-component to enable it to accept an accessible description.

Motivation and Context

Without this change, the <md-toggle-switch /> component could not provide an accessible description because the aria-describedby property takes the ID of another element to point to. In some cases the description label will be outside the components' shadow DOM, in which case there was no suitable mechanism for providing an accessible description. To circumvent this issue, this PR adds an invisible description label inside the shadow DOM of the component, and if the description property is set, then the underlying <input /> within the component will point to this invisible description label.

How Has This Been Tested?

Screenshots:

There are no visual changes to the component with this change. Screenshots show changes in what is read out by Mac VoiceOver when component is focused.

Before (If applicable):

After:

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Documentation update
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

Checklist:

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

momentum-design / momentum-ui