macOS: .m4 and .svelte files missing from Finder document associations

**Missing**

`build/lib/electron.ts` registers a curated list of extensions as macOS bundle document
types (`CFBundleDocumentTypes`), but `.m4` (m4 macro files) and `.svelte` (Svelte
components) are not in it. Finder therefore doesn't offer "Open With Code" or let users
set Code as the default handler for these files.

**Expected**

Both extensions should be registered like the other file types in the list, e.g.:

```ts
darwinBundleDocumentType(['m4'], 'default', 'm4 macro file'),
// ...
darwinBundleDocumentType(['svelte'], 'default', 'Svelte source file'),