[RFC] Expose target and browser to process.env

[louisgv]

How do you envision this feature/change to look/work like?

process.env.PLASMO_TARGET === "chrome-mv3"

process.env.PLASMO_BROWSER === "chrome"

process.env.PLASMO_MANIFEST_VERSION === "mv3"

What is the purpose of this change/feature? Why?

https://discord.com/channels/946290204443025438/1006923584142315542/1006923592069554176

(OPTIONAL) Example implementations

No response

(OPTIONAL) Contact Details

No response

Verify canary release

• [X] I verified that the issue exists in plasmo canary release

Code of Conduct

• [X] I agree to follow this project's Code of Conduct
• [X] I checked the current issues for duplicate problems.