Add proc handler for javascript event

[exeldro]

Description

Add proc handler for javascript event to a browser source

Motivation and Context

Allow other plugins to use javascript events on browser sources. This can be used by the Browser Transition plugin to send start transition event including duration

How Has This Been Tested?

With test code in the Browser Transition plugin

Types of changes

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

Checklist:

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