Add BrowserFileLoader

[camillobruni]

Similar to ShellFileLoader introduce a BrowserFileLoader as mid-term solution to clean up the file loading code.

• Move common helpers and caches from the driver to the BrowserFileLoader.
• Pass along a "counter" param where easily possible

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[camillobruni]

Mid to long term I want to fix:

• Loading the same resource as file and and preload (see js-token js-tokens: use preload source code #228 PR that is currently blocked on this)
• Clean up retry loading (I have a hunch that we can remove it but I'm not 100% certain)
• Merge more shell and browser file loading code