Provide info as to why different providers are not available

[blast-hardcheese]

An extension to #2.

Currently, we have

dstewart$ upm list-languages
python3-poetry
python3-pip  (unavailable)
bun
nodejs-npm
nodejs-pnpm
nodejs-yarn  (unavailable)
ruby-bundler
elisp-cask  (unavailable)
dart-pub  (unavailable)
java-maven  (unavailable)
rlang  (unavailable)
dotnet  (unavailable)
rust  (unavailable)
php-composer  (unavailable)

what would be preferable would be

python3-poetry
python3-pip  (unavailable, see: https://pip.pypa.io/en/stable/installation/)
bun
nodejs-npm
nodejs-pnpm
nodejs-yarn  (unavailable, see: https://yarnpkg.com/getting-started/install)
ruby-bundler
elisp-cask  (unavailable, see: https://cask.readthedocs.io/en/latest/guide/installation.html)

or even better,

dstewart$ upm list-languages
python3-poetry
bun
nodejs-npm
nodejs-pnpm
ruby-bundler

Unavailable
---
python3-pip  (see: https://pip.pypa.io/en/stable/installation/)
nodejs-yarn  (see: https://yarnpkg.com/getting-started/install)
elisp-cask   (emacs: ✓, cask: ✗, see: https://cask.readthedocs.io/en/latest/guide/installation.html)

...