Added folding on header lines

[WalrusGumboot]

When I find myself working on longer documents, I often enjoy maintaining a sense of structure by collapsing big blocks of the text at once. This PR adds some very basic functionality of doing so with folding ranges; it detects headings solely based on the '=' character. While this isn't ideal (something based on a syntactic analysis to fold all nodes that are a child of a certain heading would be more robust) it suits my personal needs well enough and might be worth considering.

A quick note on the behaviour

In a document like

= this is a heading

some text

== subheading a

some more text

== subheading b

yet more text

folding ranges will be added as such:

┌──> │ = this is a heading
│    │ 
│    │ some text
└──< │ 
┌──> │ == subheading a
│    │ 
│    │ some more text
└──< │ 
┌──> │ == subheading b
│    │ 
└──< │ yet more text

instead of the hierarchical

┌──> │ = this is a heading
│    │ 
│    │ some text
│    │ 
│ ┌> │ == subheading a
│ │  │ 
│ │  │ some more text
│ └< │ 
│ ┌> │ == subheading b
│ │  │ 
│ └< │ yet more text
└──< │ 

[Myriad-Dreamin]

related #165

[Myriad-Dreamin]

@WalrusGumboot we are reaching a nice implementation, but unluckily I guess there is a bug when we simply use document_symbol. The result of document_symbol contains all symbols in a document. Therefore, in addition to headings, labels <label> and let binding #let a = 1 and also separate a folding range here.

typst-lsp/src/server/symbols.rs

Lines 34 to 56 in 4867da4

	SyntaxKind::Label => {
	let ast_node = node
	.cast::<ast::Label>()
	.ok_or_else(|| anyhow!("cast to ast node failed: {:?}", node))?;
	let name = ast_node.get().to_string();
	if let Some(query) = query_string {
	if !name.contains(query) {
	return Ok(None);
	}
	}
	let symbol = SymbolInformation {
	name,
	kind: SymbolKind::CONSTANT,
	tags: None,
	deprecated: None, // do not use, deprecated, use `tags` instead
	location: Location {
	uri: uri.clone(),
	range: typst_to_lsp::range(node.range(), source, position_encoding).raw_range,
	},
	container_name: None,
	};
	Ok(Some(symbol))
	}

imo we should exclude them.

Could you exclude these unrelated symbols?

[WalrusGumboot]

Oops, didn't test extensively enough 😅

I guess filtering for SymbolKind::NAMESPACE works, and semantically it makes sense to fold on namespace-type symbols anyway.

[Myriad-Dreamin]

We have correct heading to folding now. But I find another bug, unfortunately. header foldings should be closed by their current parent, but now we don't and cannot do it simply. Accurately, a correct folding range identification should also regard braces. Would you like to continue solving this inperfect? I personally think it doesn't bother much.

---

Oops, the default folding range support on braces is disabled since this PR.

[WalrusGumboot]

Hmm, yeah, that seems like more work than I've got time for at the moment. Maybe after the exam season.

[Myriad-Dreamin]

well, we may reserve this feature PR for you, @WalrusGumboot. You're free to reopen it later.