Replace local type declarations for polyfill APIs with those imported from the polyfills.

[bicknellr]

This PR removes the local types describing the polyfill APIs in favor of the new TS externs from the polyfill repos.

• Add TS externs. webcomponents/polyfills#457
• Remove local tarball and use released polyfill packages instead.

[github-actions]

📊 Tachometer Benchmark Results

Summary

callback

• shack: unsure 🔍 -1% - +2% (-0.23ms - +0.57ms)
  ^{this-change vs master}

Results

shack

• Browser: chrome-headless 92.0.4515.107
• Sample size: 330
• Built by: Benchmarks #40

• Commit: a51adb4

Version	Avg time	vs this-change	vs master	vs previous-2.x-release
this-change	24.66ms - 25.20ms	-	unsure 🔍 -1% - +2% -0.23ms - +0.57ms	unsure 🔍 -0% - +2% -0.08ms - +0.61ms
master	24.47ms - 25.06ms	unsure 🔍 -2% - +1% -0.57ms - +0.23ms	-	unsure 🔍 -1% - +2% -0.27ms - +0.46ms
previous-2.x-release	24.46ms - 24.88ms	unsure 🔍 -2% - +0% -0.61ms - +0.08ms	unsure 🔍 -2% - +1% -0.46ms - +0.27ms	-

_{tachometer-reporter-action v2 for Benchmarks}

[bicknellr]

For some reason, I can't add @sorvell as a reviewer.

[kevinpschaaf]

LGTM, just the one question about ShadowRoot.

[kevinpschaaf]

Why do we still need ShadowRoot on window? Seems like that's in lib.dom.d.ts...

[bicknellr]

I was limiting the PR to only removing the parts that are now provided externally, but it builds without a lot of other stuff in here. Should I remove those also?

[bicknellr]

(FWIW, this declaration of ShadowRoot is not necesary, but the part below that adds adoptedStyleSheets still is.)

[bicknellr]

Actually, I think I'd rather wait to clean unrelated extra declarations out of here in another PR.

[bicknellr]

Side note: Now that I think about it, lit/lit#1904 was probably a mistake that will become repairable once this PR (edit: and a PR removing the other unnecessary parts) is in. Originally, I found that those types were conflicting internally with something (the original JS externs for the polyfills, I think?) so that PR avoided the problem by not adding to Window. Now that the polyfill types all have exactly one definition, it might be better to go back to adding Lit-specific special properties to Window because they are actually there and a collision would indicate a real problem.

[bicknellr]

(We talked in a meeting about this, I'm going to go ahead and merge.)