[@next] port deprecated (2.x) button for backwards compatibility

[DRiFTy17]

This change brings support for the "decorator" pattern for buttons that we used in the 2.x version to the 3.0 version. This is only for backwards compatibility and should be API-compatible with the 2.x implementation.

It allows for slotting native <button> elements in for backwards compatibility to ease the upgrade process:

<forge-deprecated-button>
  <button type="button">Decorated button</button>
</forge-deprecated-button>

This was ported from the 2.x version and integrated with design tokens (it uses the same styles from the new 3.0 button but with ::slotted() selectors instead and some minor overrides to match 2.x.

This is not a direct port from 2.x and it wasn't intended to be. The styles were moved to the shadow DOM, and it was integrated with the new state-layer and focus-indicators. This is purely for supporting slotted native <button> elements.

Consumers will need to opt-in to renaming their existing elements to <forge-deprecated-button> if they want to avoid sweeping their code.

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[MikeMatusz]

Looks good. Probably should throw an @deprecated JSDoc tag on the component class/interface and define method. Curious if there'd be a good way to propagate that to the Angular modules via the schematic, too.

[github-actions]

🚀 PR was released in v3.0.0-next.17 🚀