feat: createContentScriptUi helper

[aklinker1]

This closes #139. See demo project for example usage:

wxt/demo/src/entrypoints/ui.content/index.ts

Lines 9 to 22 in 78dccc3

	const ui = await createContentScriptUi(ctx, {
	name: 'demo-ui',
	type: 'inline',
	append: 'before',
	anchor: 'form[role=search]',
	mount: (uiContainer) => {
	const app = document.createElement('div');
	app.textContent = 'Custom content script UI';
	uiContainer.append(app);
	},
	});
	ui.mount();
	setTimeout(ui.remove, 5000);

Todo

• Basic implementation
• Unit test createContentScriptUi
• Automatically add cssInjectionMode: !"manifest" content script CSS files to manifest web_accessible_resources
• Only add styles if cssInjectionMode: "ui"
• Add inline docs to all the new functions and types

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[codecov]

Codecov Report

Merging #143 (8b3601f) into main (61e57b7) will increase coverage by 0.80%.
The diff coverage is 99.51%.

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   84.45%   85.26%   +0.80%     
==========================================
  Files          52       61       +9     
  Lines        3995     4615     +620     
  Branches      366      426      +60     
==========================================
+ Hits         3374     3935     +561     
- Misses        621      680      +59     

Files	Coverage Δ
src/client/createContentScriptUi.ts	100.00% <100.00%> (ø)
src/client/index.ts	100.00% <100.00%> (ø)
src/client/utils/ContentScriptContext.ts	74.73% <100.00%> (ø)
src/core/types/external.ts	100.00% <100.00%> (ø)
src/core/utils/manifest.ts	71.84% <100.00%> (+4.28%)	⬆️
vitest.setup.ts	100.00% <100.00%> (ø)
src/core/server.ts	22.14% <66.66%> (+1.55%)	⬆️

... and 5 files with indirect coverage changes

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