Respect user's preferred browser

[Sharparam]

Problem Description

Pdoc uses a hardcoded priority list for which browser to launch, ignoring the user's preference if set in the BROWSER environment variable or similar.

Proposal

Pdoc should launch the browser specified by $BROWSER, if set, or use something like xdg-open (which will take $BROWSER into account).

Alternatives

Place Firefox higher up in the priority, users with Firefox installed are more likely to actively prefer it over other browsers. Users preferring Chrom(e|ium) are less likely to have Firefox installed (probably).

Additional context

Hardcoded list here: https://github.com/mitmproxy/pdoc/blob/main/pdoc/web.py#L173-L186

Reproduction steps

export BROWSER=firefox
pdoc <module>
# Expect docs to open in already running firefox, pdoc instead spawns a chromium process if it is installed

[Sharparam]

Workaround for people not using Ubuntu (hijacking x-www-browser to run xdg-open):

Create a script in your path (e.g. ~/.local/bin/x-www-browser:

exec xdg-open "$@"

Since x-www-browser is higher in the priority, this will now correctly respect browser choice as resolved by XDG.