Improve performance of runSnippetCursor

[shunby]

Optimizes runSnippetCursor in run_snippet.ts by reducing redundant calls to view.state.sliceDoc.
Partially solves #320 by improving the editting of large files.

Performance Testing

Tested with a large markdown file generated with the following command:

python -c "print('aa\n'*4000)" > test.md

Pushed a key when the cursor is at the bottom of the file and measured the time spent handling the keydown event using DevTools.

Test Environment:

• OS: Microsoft Windows 11 Home
• CPU: Intel(R) Core(TM) i7-8750H CPU
• RAM: 16GB

Results:

• Before the update: Handling a single keydown event took approximately 50ms, with runSnippetCursor consuming 47ms.
• After the update: Handling a single keydown event took approximately 3ms, with runSnippetCursor consuming 0.2ms.

[artisticat1]

Thanks!

[CamWam]

Thank you!!!!