Add function to signal the browser source from JavaScript

[exeldro]

Description

adds void source_browser_signal(ptr source, string signal) to the global signal handler
adds void browser_signal(ptr source, string signal) to the signal handler of the browser source
in javascript you can signal a signal handler like this: window.obsstudio.signal("signal")

Motivation and Context

Allow JavaScript to send signals to plugins, without the need for an obs websocket connection
More generic solution replacing #423

How Has This Been Tested?

On windows 11 with changes to the markdown source to allow it to resize it self

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.