docs: update browser support table for FAST Element

[janechu]

🙋 Documentation Request

We should update our browser support matrix to allow for the use of newer features and provide guidance for polyfilling if necessary. This may lead to updating target in the tsconfig and removing built in fallbacks (e.g. adoptedStylesheets, globalThis, requestIdleCallback).

[chrisdholt]

My $0.02 -

It is great that we support all the way back to Safari 15.4, but this comes at a cost with the reality of where modern browsers are today. Top of mind for things that get baked-in today is support for previous implementations, or the lack of implementation of Adopted StyleSheets. We handle the scenario internally by setting a given style strategy based on support, but ideally we can move supporting this into a polyfill or "patchable" approach that could be bundled separately. I believe that in Safari 17.1 the AdoptedStyleSheet issue present in the 16.4 launch is resolved, which would allow us to handle most modern implementations without the heavy-handed fallback or adjustments for implementation deltas. I'd take the same approach for things pre 17.1 which can be polyfilled or plumbed outside the core bundle, rather than internalized.