[Feature Request] Support @inject-into for userscripts

[theacrat]

Is your feature request related to a problem? Please describe.
I'm trying to use some scripts that use @inject-into content, as some websites such as Twitter have CSP configured to block scripts appended to head.

Describe the solution you'd like
Support the @inject-into option to allow userscripts to use the content script context.

Describe alternatives you've considered
N/A

Additional context
N/A