[Enhancement] Enable strictTemplate checks

[RasmusKjeldgaard]

• I have written a good issue

Describe the enhancement

The fullTemplateTypeCheck flag that we are using in our library configuration (tsconfig.lib.json) will be deprecated in Angular 13, and be replaced by the strictTemplates flag.

With this check enabled in our current repo, we will see errors where we try to use strings for inputs that are actually typed, e.g. 'sm' for the size parameter on the button, that has the type ButtonSize.
Same goes for ThemeColor, other sizes like AvatarSize and so on.

I think in many of these cases, it should be possible to use a string as input value, but that is not possible if we or any projects enable the strictTemplates flag, which will probably be the case for almost everyone when upgrading to Angular 13.

Describe the solution you'd like

Change the enum to a string union-type or a union of strings and the specific size/themecolor enums that already exist.
Beware of breaking changes in this regard.

Have you considered any alternatives?

Are there any additional context?

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