Add folding range lsp support

[ssmifi]

Closes #26
and implements the folding feature, which is basically a port from Langium.

Customization can be done via the FoldingRangeFilter interface.

[msujew]

Thanks, I like the filter strategy approach! A few suggestion, see below 👍

[msujew]

Suggestion: Why pass the fixture if all you need is t anyway (which already exists in Doc)?

[msujew]

Suggestion: Instead of the test passing in the folding ranges, the method should retrieve them by itself. This keeps the tests much smaller. See as an example:

fastbelt/test/doc_fixture.go

Lines 104 to 122 in ce62095

	func (d *Doc) AssertDefinition(label string) {
	d.fixture.t.Helper()
	location, err := d.markerLocation(label, false)
	if err != nil {
	d.fixture.t.Fatalf("fbtest: %v", err)
	}
	defProvider, err := service.Get[server.DefinitionProvider](d.fixture.sc)
	if err != nil {
	d.fixture.t.Fatalf("fbtest: no definition provider available: %v", err)
	}
	links, err := defProvider.HandleDefinitionRequest(d.fixture.ctx, &lsp.DefinitionParams{
	TextDocumentPositionParams: lsp.TextDocumentPositionParams{
	TextDocument: lsp.TextDocumentIdentifier{URI: d.Document.URI.DocumentURI()},
	Position: location.LspPosition(),
	},
	})
	if err != nil {
	d.fixture.t.Fatalf("fbtest: definition request failed: %v", err)
	}

[ssmifi]

But then I can pass the fixture, yes? It's needed for the renameProvider anyways, right?

[msujew]

It's not required. But it's alright 👍