Add microbenchmarks, improve RangeMap perf

[rbuckton]

This adds microbenchmarks to analyze the performance of several parts of Deopt Explorer and improves the performance of RangeMap, which is used throughout the extension to collection information related to positions and ranges, such as for producing hovers.

The key changes were to support a user-defined comparer in SplayTree, which allowed RangeMap to switch from using series of nested SplayTrees that map startLine -> startCharacter -> endLine -> endCharacter -> T to a single SplayTree whose keys are Ranges using a custom comparer. The ranges are sorted by range.start in ascending order, then range.end in descending order. This order allows us to treat subsequent ranges as if they were contained by prior ranges as long as they share the same start position, which results in more efficient traversal.

This also adds linked list-like pointers to SplayTree.Node to make in-order traversal more efficient and reliable.

The net result is a 40x improvement in calls to .findAllContaining():

[jakebailey]

Long-term, I wonder if you'd have it easier running vitest and its built-in benchmarking; I've found it to be pretty handy and avoids a lot of boilerplate.

[rbuckton]

At this point I'm pretty familiar with jest's quirks. I'm not sure I want to switch to a different test framework just for benchmarking.

My biggest issue at the moment is bit rot with my CI builds..