ark: Implement folding rules

[kevinushey]

VSCode allows an extension to declare folding rules within language-configuration.json:

https://code.visualstudio.com/api/language-extensions/language-configuration-guide#folding

However, it's fairly rigid -- each foldable region must have separate (and uniquely-matched) start and end boundaries. However, we'd likely want to support foldable sections -- for example:

# Section 1 ----

## Section 1.1 ----

## Section 1.2 ----

For that reason, we'll probably need to implement folding rules in the LSP (unless we want to patch the VSCode behavior here)

[kevinushey]

This probably isn't necessary for MVP, if we want to defer this to later.

[EmilHvitfeldt]

Since we are having quarto support, it would be nice if the folding rules also worked with fenced divs: https://quarto.org/docs/authoring/markdown-basics.html#divs-and-spans

In the following picture, it would be neat if line 19-23 could collapse, 26-30 could collapse and 17-32 could collapse

For clarity, i made the outside div with 4 colons, but that isn't strictly needed by quarto, so the columns work on a first-in-first-out kinda basis when they are created with the same number of colons

[petetronic]

Here is RStudio IDE's guide to Code Sections:
https://docs.posit.co/ide/user/ide/guide/code/code-sections.html#code-sections
@kevinushey was this the functionality you had in mind?

[petetronic]

We recently discussed modifying the newly added positron-code-cells extension to allow runtimes to better control code ranges and behaviors. However, we've not yet prioritized this work.

[petetronic]

Since we are having quarto support, it would be nice if the folding rules also worked with fenced divs: https://quarto.org/docs/authoring/markdown-basics.html#divs-and-spans

In the following picture, it would be neat if line 19-23 could collapse, 26-30 could collapse and 17-32 could collapse

Hi @EmilHvitfeldt, for Quarto docs, the folding rules would be controlled by the Quarto extension. Would you mind logging that request with their backlog here? https://github.com/quarto-dev/quarto/issues

[EmilHvitfeldt]

another use-case here is being able to fold test_that() blocks in R package tests, (missing this one right now). May be related to #1428.