perf(lsp): Cache semantic tokens for open documents (#23799)

VScode will typically send a `textDocument/semanticTokens/full` request
followed by `textDocument/semanticTokens/range`, and occassionally
request semantic tokens even when we know nothing has changed. Semantic
tokens also get refreshed on each change. Computing semantic tokens is
relatively heavy in TSC, so we should avoid it as much as possible.

Caches the semantic tokens for open documents, to avoid making TSC do
unnecessary work. Results in a noticeable improvement in local
benchmarking

before:
```
Starting Deno benchmark
-> Start benchmarking lsp
   - Simple Startup/Shutdown 
      (10 runs, mean: 383ms)
   - Big Document/Several Edits 
      (5 runs, mean: 1079ms)
   - Find/Replace
      (10 runs, mean: 59ms)
   - Code Lens
      (10 runs, mean: 440ms)
   - deco-cx/apps Multiple Edits + Navigation
      (5 runs, mean: 9921ms)
<- End benchmarking lsp
```

after:
```
Starting Deno benchmark
-> Start benchmarking lsp
   - Simple Startup/Shutdown 
      (10 runs, mean: 395ms)
   - Big Document/Several Edits 
      (5 runs, mean: 1024ms)
   - Find/Replace
      (10 runs, mean: 56ms)
   - Code Lens
      (10 runs, mean: 438ms)
   - deco-cx/apps Multiple Edits + Navigation
      (5 runs, mean: 8927ms)
<- End benchmarking lsp
```

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