Remove syntax_tree gem, add YARP gem

[andyw8]

As discussed in today's meeting, we'll use a feature branch (I've named it yarp) for the YARP migration, to allow us to review PRs is smaller chunks.

We won't expect a CI pass until everything is complete.

[github-actions]

Benchmark results in seconds (slowest at top)
textDocument/completion average: 0.262348 std_dev: 0.004905
textDocument/diagnostic average: 0.042041 std_dev: 0.01073
textDocument/definition average: 0.005433 std_dev: 0.003116
textDocument/selectionRange average: 0.00415 std_dev: 0.000623
textDocument/documentHighlight average: 0.002591 std_dev: 0.000233
textDocument/codeLens average: 0.002503 std_dev: 0.000216
textDocument/semanticTokens/full average: 0.002489 std_dev: 0.00034
textDocument/documentLink average: 0.002485 std_dev: 0.000254
textDocument/documentSymbol average: 0.00244 std_dev: 0.000211
textDocument/foldingRange average: 0.002375 std_dev: 0.000162
textDocument/semanticTokens/range average: 0.001635 std_dev: 8.3e-05
codeAction/resolve average: 0.001458 std_dev: 0.000111
textDocument/hover average: 0.001396 std_dev: 0.000108
textDocument/inlayHint average: 0.001384 std_dev: 0.000116
textDocument/onTypeFormatting average: 0.000846 std_dev: 8.6e-05
textDocument/formatting average: 0.000834 std_dev: 0.000298
textDocument/codeAction average: 0.000816 std_dev: 8.5e-05
================================================================================
Comparison with main branch:
textDocument/semanticTokens/full unchanged
textDocument/semanticTokens/range unchanged
textDocument/documentSymbol unchanged
textDocument/foldingRange unchanged
textDocument/formatting unchanged
textDocument/diagnostic unchanged
textDocument/documentLink unchanged
textDocument/inlayHint unchanged
textDocument/selectionRange unchanged
textDocument/documentHighlight unchanged
textDocument/hover unchanged
textDocument/codeAction unchanged
textDocument/onTypeFormatting unchanged
codeAction/resolve unchanged
textDocument/completion unchanged
textDocument/codeLens unchanged
textDocument/definition unchanged
================================================================================
Missing benchmarks:
RubyLsp::Requests::ShowSyntaxTree

[st0012]

Looks good, but do we want to wait a day for ruby/prism#1214?

[andyw8]

Looks good, but do we want to wait a day for ruby/yarp#1214?

I don't think it's a big deal to switch later - I've added a task to #449 as a reminder.