Provide --update-server command from executable

[vinistock]

The relationship between the global ruby-lsp executable version vs the server version that actually gets run is often a source of confusion to users. Additionally, if some other gem is constraining our upgrades, users sometimes have trouble identifying which gem is holding back the upgrade.

On top of those, users with relative paths in their Bundler settings don't have a very easy way to upgrade because Bundler considers the Gemfile path to be the root of everything. If you have BUNDLE_PATH=vendor/bundle, then running this

BUNDLE_GEMFILE=.ruby-lsp/Gemfile bundle update ruby-lsp

Will install gems in .ruby-lsp/vendor/bundle because the path becomes relative to the Gemfile. All of this is not great and we can provide better tools for upgrades and diagnosing issues.

Let's add a ruby-lsp --update-server command that will simply run bundle update using our composed environment, with all Bundler settings turned into absolute paths (which is what we do while regular composition). Furthermore, if a gem is preventing the upgrade, it would be awesome if we could inform the user about which one it is.