feat: prefix content script names

[ExposedCat]

Overview

Currently, only unique-name.content can be used for naming content scripts. This results in inconvenient sorting in IDEs, such as:

- entrypoints
  - google.content
  - popup
  - safari.content

This PR allows to use content.unique-name format, so we would have

- entrypoints
  - content.google
  - content.safari
  - popup

which essentially groups content scripts in places such as IDE file viewers

Manual Testing

Create any content script at entrypoints/content.name/index.ts or entrypoints/content.name.ts

Related Issue

N/A

This PR closes #2043

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[aklinker1]

Thanks for taking the time to create a PR, but I don't want to change the entrypoint patterns right now. See #2043 (comment) for more details.