Fix runnables code lens in multi-file sway projects

[sdankel]

Description

Closes #4983

The issue was that we were returning all of the runnables for the workspace, rather than just the file. We were also calling get_range_from_span for every runnable at request time, which @JoshuaBatty discovered is very costly.

• The runnables map is now mapping [file path] => [vector of runnables for that file]
• Rather than iterating over all of the runnables in the session, we just fetch the vector of runnables indexed by the file path.
• Runnables are now storing the range rather than the span, so we don't need to convert it at request time.
• Added another test that expects no runnables to be returned for a file in a workspace that has runnables in other files. This test was failing before the change. Previously, the runnables test was running on test files from the e2e directory, so I copied that into the LSP fixtures.
• Moved the code_lens logic into a capability and updated the bencher

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[JoshuaBatty]

Nice optimisation! just some small nits.