refactor: resolve browser via env variables based on build rather than user agent

[hoxyq]

Fixes #26911, #26860.

Currently, we are parsing user agent string to determine which browser is running the extension. This doesn't work well with custom user agents, and sometimes when user turns on mobile dev mode in Firefox, we stop resolving that this is a Firefox browser, extension starts to use Chrome API's and fails to inject.

Changes:
Since we are building different extensions for all supported browsers (Chrome, Firefox, Edge), we predefine env variables for browser resolution, which are populated in a build step.

[ansh-ag]

Im not sure if this is the correct place to post this but curious to discuss,
shouldn't our extensions be the same for all chromium based browser ? why should we have separate extension for edge or chrome in that case ? Wouldn't that cover a bigger pool of browsers ?

[hoxyq]

Im not sure if this is the correct place to post this but curious to discuss, shouldn't our extensions be the same for all chromium based browser ? why should we have separate extension for edge or chrome in that case ? Wouldn't that cover a bigger pool of browsers ?

Yes, essentially Chrome and Edge extensions are almost the same. There are some small differences, like in manifest files, where we need to provide text description of the extension, which can be customised for different browsers, or setting icons logic in runtime.