Handle BROWSER env var for the various `forge browse` commands

[kfdm]

Right now it has a few options coded

https://github.com/git-pkgs/forge/blob/v0.3.0/internal/cli/browse.go#L90-L106

Python handles several extra by checking things like TERM, DISPLAY, and BROWSER

https://github.com/python/cpython/blob/3.14/Lib/webbrowser.py#L536-L580

I wonder if at least BROWSER support could be added. A quick scan didn't really show any existing packages that handle BROWSER. There is an internal version used in Go that could possibly be referenced

https://cs.opensource.google/go/go/+/refs/tags/go1.25.6:src/cmd/internal/browser/browser.go

But of course the internal scope means it can't be called from regular go projects.

Any thoughts on a version of this that you would be happy with if I was to attempt at PR ?