feat: improve UX handling when multiple main files detected

[nilayarya]

Description of Change

Suggests UI changes to resolve #1508

1. New MenuItem 'Set as Main Entry Point' available on right-click of valid main files (main.js, main.cjs, main.mjs)
2. A Popup if there are multiple main files in the loaded fiddle (shows up only once a session )

Some gifs:

gist-url: https://gist.github.com/nilayarya/dfac02c9af5c699f3efc146ddf946f9f

CC @dsanders11

Checklist

• PR description included and stakeholders cc'd
• This PR was not created with AI. (PRs created mainly with AI will be closed. They waste our team's time. We ban repeat offenders.)
• yarn test passes
• tests are changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

[coveralls]

coverage: 87.482% (+0.01%) from 87.472%
when pulling 9c9e60d on nilayarya:multi-main
into d5e133f on electron:main.

[BlackHole1]

Hi @nilayarya, thank you for your PR, but it seems to be incomplete. From the current PR, I can only temporarily consider this as an improvement for the edge case when there are multiple mainEntryPoints in the gist. When I create another mainEntryPoint from the left sidebar, it throws an error. According to the description in #1508, I believe @dsanders11 is hoping for a complete mainEntryPoint switching mechanism.

fiddle/src/renderer/editor-mosaic.ts

Lines 284 to 290 in 3838dd5

	const entryPoint = this.mainEntryPointFile();
	if (isMainEntryPoint(id) && entryPoint) {
	throw new Error(
	`Cannot add file "${id}": Main entry point ${entryPoint} exists`,
	);
	}

[BlackHole1]

Suggested change

	if (!this.mainEntryPoint || !this.files.get(this.mainEntryPoint)) {
	const entryPoint = Array.from(this.files.keys()).find((id) =>
	isMainEntryPoint(id),
	);
	if (entryPoint) this.mainEntryPoint = entryPoint;
	return entryPoint;
	}
	return this.mainEntryPoint;
	public mainEntryPointFile(): EditorId | null {
	if (this.mainEntryPoint && this.files.get(this.mainEntryPoint)) {
	return this.mainEntryPoint;
	}
	for (const [id] of this.files) {
	if (isMainEntryPoint(id)) {
	this.mainEntryPoint = id;
	break;
	}
	}
	return this.mainEntryPoint;

Make the code more intuitive and optimize performance a bit.