Allow source maps files in webextension

[SpaceK33z]

↪️ Pull Request

Fixes #8530 — there was no way to generate source map files when you are making a web extension.

💻 Examples

With this fix it still defaults to inline source maps, but you can overwrite it:

"sourceMap": {
  "inline": false,
  "inlineSources": false
}

🚨 Test instructions

There don't seem to be tests for this part of the code, but with the example above you can easily see that it generates .map files when using that config with this Parcel config:

{
  "extends": "@parcel/config-webextension"
}

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs