fix: Avoid autoreloading by Chrome-generated files

[Rob--W]

Fixes #3468

Must be merged after #3697 to avoid test failures.

[Rob--W]

CI was repeatedly failing due to onChange called for the parent directory in the test:

  1) watcher
       watcher ignores _metadata
         ignores change to non-toplevel _metadata directory (await):
     expected spy to not have been called but was called once

I identified the root cause, to be a watchpack bug, and created a PR to fix that: #3697.

CI is currently red because #3697 is not included in the PR.

Here is a fully green CI job for #3697 + this PR: https://app.circleci.com/pipelines/github/mozilla/web-ext/4894/workflows/871b8d37-b0ce-475a-9094-182c9c889cb0

[Rob--W]

While this PR works, I wonder whether I should have added the ignore patterns to FileFilter instead:

web-ext/src/util/file-filter.js

Lines 32 to 39 in 3b2a518

	baseIgnoredPatterns = [
	'**/*.xpi',
	'**/*.zip',
	'**/.*', // any hidden file and folder
	'**/.*/**/*', // and the content inside hidden folder
	'**/node_modules',
	'**/node_modules/**/*',
	],

That is passed to the watcher at:

web-ext/src/extension-runners/index.js

Lines 199 to 206 in 3b2a518

	const fileFilter = createFileFilter({ sourceDir, artifactsDir, ignoreFiles });
	return onSourceChange({
	sourceDir,
	watchFile,
	watchIgnored,
	artifactsDir,
	onChange: () => reloadExtension(sourceDir),
	shouldWatchFile: (file) => fileFilter.wantFile(file),