Basic Definition Finder for IDE

[Myriad-Dreamin]

This PR introduces a typst_ide::definition function. Its signature simulates that of typst_ide::tooltip. With given a cursor, typst_ide::definition finds the definition of item at cursor:

• DefinitionKind::Module: any module definitions with a span.
• DefinitionKind::Item: any variables with a span or any builtin items (with a detached span).
• DefinitionKind::Label: any <label> definitions with a span.

typst_ide::definition doesn't find:

• a label in bibliography, whose name doesn't have a span, see Spanned diagnostics for non-Typst files #563.
• the real span of definition of an imported items, since we need to change the structure of scopes and tracer. I think @laurmaedje can design and improve them better than me.
• a definition of field, same reason as above.

Instead of completeness, we may focus on setting up workflow for typst_ide::definition.

• I added two common matchers for all ide functions. The design of these matchers might be arguable.
• I find we cannot test the functionality of finding definitions in other files well, since TestWorld provides an only main source file.

[laurmaedje]

Thanks for taking this on! It's really needed. Also, sorry for the long wait time. I left some comments now.

[Myriad-Dreamin]

Resolved all discussion other than the one to add a README.md.

[laurmaedje]

I refactored a little bit, but got some new questions in the process.

An aside: The node.find(some_expr.span()) is an interesting trick/workaround for the lack of a LinkedNode-style typed AST! ^^

[Myriad-Dreamin]

I have updated the PR according to the last conversation.

[laurmaedje]

Thank you!