Enable support for browser_specific_settings in Firefox

[SebastienGllmt]

Firefox supports browser_specific_settings as an alternative to applications because Chrome rejects addons with applications inside the manifest but supports the browser_specific_settings field.

This was originally introduced in here
And you can see it inside the Firefox codebase here
And supported by the Addons-linter here

Both Firefox and the addons-linter give warnings if both applications and browser_specific_settings are present and give priority to browser_specific_settings
Similarly, my change gives priority to browser_specific_settings but I did not add a warning.

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This change is 

[SebastienGllmt]

Any chance this can be merged? If there is a reason it hasn't been merged yet let me know and I can fix it.

[jleyba]

Save the JSON.parse result to a variable so you don't have to re-parse on line 185 below.

That, or perhaps change this to:

let {applications, browser_specific_settings} = JSON.parse(buf.toString('utf8'))

[SebastienGllmt]

@jleyba the code was like that to match the original Firefox code but it seems they've cleaned this up themselves. I pushed a new commit based on the Mozilla linter logic: https://github.com/mozilla/addons-linter/blob/master/src/parsers/manifestjson.js#L277

[SebastienGllmt]

Just another ping on this issue 🙏