Fix activation of linters on unopened files caused by markdown plugin

[solomatov]

Here's the issue: #50874

Solution:

• Bug was caused by invocations of vscode.workspace.openTextDocument during indexing process
• I create a document index which maintains a map from uri to TextDocument
• When we need a document, we
  • Check whether it's already loaded and if it is, we use loaded document
  • Otherwise we read it through fs API

[msftclas]

All CLA requirements met.

[solomatov]

@mjbvz Do you have any other things to fix in the review?

[mjbvz]

This is going in the correct direction but want to make sure we don't make the more common uses of TableOfContentsProvider more inefficient. Let me know if you have any questions about the comments

[solomatov]

@mjbvz It seems that I fixed all your suggestions. Thanks for them, they made the code better and much shorter.

[mjbvz]

Thanks for the PR and for the changes! Looks good to me.

This will be in the next VS Code insiders build and is scheduled to ship with VS Code 1.39