Polyfill is included in all scripts, even if they will not be run in an extension context

[oliverdunk]

Currently, we add the browser polyfill to all files ending in .ts. This works nicely to make sure it's available in content scripts, but has limitations. Namely, adding a script tag to the page and referencing a JS file from the extension will try to load the polyfill, and fail because the script is being loaded in the normal world and without access to browser APIs.

We should probably make which files to polyfill configurable or do some sort of AST search for a call to browser.

[oliverdunk]

Starting looking at this. @typescript/sandbox does provide a way of getting the AST, but it only returns it for the current program, rather than letting you pass in individual files. This seems to cause some errors to be thrown that I need to look in to further.