Add 'overrides' key for browser-specific manifest settings

[tbrockman]

Details

Add overrides key to manifest to allow setting browser-specific manifest key-values, using just a dumb shallow-merge.

I personally needed this feature because Firefox CSP automatically upgrades HTTP connections to HTTPS, but setting anything else broke functionality in Safari.

Related PRs:

• https://github.com/PlasmoHQ/examples/pull/67
• https://github.com/PlasmoHQ/plasmo-constants/pull/2

Code of Conduct

• [x] I agree to follow this project's Code of Conduct
• [x] I agree to license this contribution under the MIT LICENSE
• [x] I checked the current PR for duplication.

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