feat(runtime): support already-existing shadow roots

[alicewriteswrongs]

This adds some checks when a Stencil component is booting up that will
re-use a shadow root if the host element already has one, instead of
just calling this.attachShadow in all cases.

Additionally, Stencil's virtual DOM will now cleanly take over any
elements which are already present in its root element when it first
runs. This will allow it to re-use elements that were already created in
the shadow root, as well as clean up any stray elements that shouldn't
be there.

Together these changes will allow the Stencil runtime to smoothly take
over a shadow root which was created via a <template> tag in the
page's HTML, i.e. via declarative shadow DOM (DSD).

NOTE this only enables the Stencil runtime to re-use a shadow root
created via DSD, it does not add any support to Stencil for rendering
a suitable <template>... HTML string.

STENCIL-1316

What is the current behavior?

GitHub Issue Number: N/A

What is the new behavior?

Documentation

Does this introduce a breaking change?

• Yes
• No

Testing

Other information

[github-actions]

--strictNullChecks error report

Typechecking with --strictNullChecks resulted in 1080 errors on this branch.

That's the same number of errors on main, so at least we're not creating new ones!

reports and statistics

Our most error-prone files

Path	Error Count
src/dev-server/index.ts	37
src/dev-server/server-process.ts	32
src/compiler/prerender/prerender-main.ts	22
src/runtime/vdom/vdom-render.ts	21
src/runtime/client-hydrate.ts	20
src/screenshot/connector-base.ts	19
src/testing/puppeteer/puppeteer-element.ts	19
src/dev-server/request-handler.ts	15
src/compiler/prerender/prerender-optimize.ts	14
src/compiler/sys/stencil-sys.ts	14
src/runtime/connected-callback.ts	14
src/sys/node/node-sys.ts	14
src/compiler/prerender/prerender-queue.ts	13
src/compiler/sys/in-memory-fs.ts	13
src/runtime/set-value.ts	13
src/compiler/output-targets/output-www.ts	12
src/compiler/transformers/test/parse-vdom.spec.ts	12
src/compiler/transformers/transform-utils.ts	12
src/mock-doc/test/attribute.spec.ts	12
src/compiler/build/compiler-ctx.ts	11

Our most common errors

Typescript Error Code	Count
TS2322	351
TS2345	330
TS18048	193
TS18047	76
TS2722	27
TS2532	24
TS2531	19
TS2454	14
TS2790	11
TS2352	9
TS2769	8
TS2416	7
TS2538	4
TS2493	3
TS18046	2
TS2684	1
TS2430	1

Unused exports report

There are 15 unused exports on this PR. That's the same number of errors on main, so at least we're not creating new ones!

Unused exports

File	Line	Identifier
src/runtime/bootstrap-lazy.ts	21	setNonce
src/screenshot/screenshot-fs.ts	18	readScreenshotData
src/testing/testing-utils.ts	198	withSilentWarn
src/utils/index.ts	145	CUSTOM
src/utils/index.ts	245	NODE_TYPES
src/utils/index.ts	269	normalize
src/utils/index.ts	7	escapeRegExpSpecialCharacters
src/compiler/app-core/app-data.ts	25	BUILD
src/compiler/app-core/app-data.ts	116	Env
src/compiler/app-core/app-data.ts	118	NAMESPACE
src/compiler/fs-watch/fs-watch-rebuild.ts	123	updateCacheFromRebuild
src/compiler/types/validate-primary-package-output-target.ts	61	satisfies
src/compiler/types/validate-primary-package-output-target.ts	61	Record
src/testing/puppeteer/puppeteer-declarations.ts	485	WaitForEventOptions
src/compiler/sys/fetch/write-fetch-success.ts	7	writeFetchSuccessSync

[github-actions]

PR built and packed!

Download the tarball here: https://github.com/ionic-team/stencil/actions/runs/9269810431/artifacts/1544211247

If your browser saves files to ~/Downloads you can install it like so:

unzip -d ~/Downloads ~/Downloads/stencil-core-4.18.2-dev.1716901330.409504a.tgz.zip && npm install ~/Downloads/stencil-core-4.18.2-dev.1716901330.409504a.tgz

[alicewriteswrongs]

closing in favor of #5792