Outline provider changes

[DonJayamanne]

Tasks

Beta Give feedback

1. Move outline provider to notebook worker
2. Given that this was slow and hanging the renderer process, we might want to measure the perf of the computation
3. Code symbols in Notebook outline is not when the text in a code cell is updated #210704
   a11y-partner +1
   

[DonJayamanne]

• Limit number of cells processed?
  Should we not have a limit on the number of code & markdown cells we process when generating the outline.
  Assume users have 1000cells and 1000md cells, surely we shouldn't try to process all of these when attempting to generate the outline.
  Just as we limit the number of cells to 100 when generating the code symbols we might want to have a limit here too.
  https://github.com/microsoft/vscode/blob/main/src/vs/workbench/contrib/notebook/browser/viewModel/notebookOutlineProvider.ts#L140

• Get text by providing a range (or offset), etc
  Given the fact that we're generating outlines a lot and its been identified to be slow, we might be able to save a few MS by avoiding the use of cell.getText() and extract only a range of text.

const fullContent = cell.getText().substring(0, 10000);

Or perhaps we can instead just get the headers line by line (using the cell model API to get the lines at a time and then using the getMarkdownHeadersInCellFallbackToHtmlTags for each line,

@rebornix /cc