Set BUNDLER_VERSION to avoid version mismatch restarts

[vinistock]

Motivation

Closes #2532

The reported bug happens on the following conditions:

1. We require "bundler" to use Bundler APIs to set up the composed bundle
2. If the required version ends up being different than the one locked in the application's lockfile (under BUNDLED_WITH), then when we run bundle exec ruby-lsp after the composed bundle setup, Bundler will force restart to load the Bundler version specified in the lockfile
3. When Bundler performs the restart, it erases everything we set as part of the environment, which makes the command fail for users who have settings like BUNDLE_PATH configured

Implementation

The implementation starts to keep track of the locked Bundler version. If there is one, we both set BUNDLER_VERSION in the environment and run all bundle commands with the specified version, which ensures that the composed lockfile under .ruby-lsp will use the same version and avoid restarts.

If the locked Bundler version is not installed, we also auto install it otherwise running bundle _x.x.x_ fails.

Automated Tests

Added a test.