[ty] Initial implementation of signature help provider (#19194)

This PR includes:
* Implemented core signature help logic
* Added new docstring method on Definition that returns a docstring for
function and class definitions
* Modified the display code for Signature that allows a signature string
to be broken into text ranges that correspond to each parameter in the
signature
* Augmented Signature struct so it can track the Definition for a
signature when available; this allows us to find the docstring
associated with the signature
* Added utility functions for parsing parameter documentation from three
popular docstring formats (Google, NumPy and reST)
* Implemented tests for all of the above

"Signature help" is displayed by an editor when you are typing a
function call expression. It is typically triggered when you type an
open parenthesis. The language server provides information about the
target function's signature (or multiple signatures), documentation, and
parameters.

Here is how this appears:


![image](https://github.com/user-attachments/assets/40dce616-ed74-4810-be62-42a5b5e4b334)

---------

Co-authored-by: UnboundVariable <unbound@gmail.com>
Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ty_ide/src/lib.rs

@@ -1,14 +1,17 @@
 mod completion;
 mod db;
+mod docstring;
 mod find_node;
 mod goto;
 mod hover;
 mod inlay_hints;
 mod markup;
 mod semantic_tokens;
+mod signature_help;
 
 pub use completion::completion;
 pub use db::Db;
+pub use docstring::get_parameter_documentation;
 pub use goto::goto_type_definition;
 pub use hover::hover;
 pub use inlay_hints::inlay_hints;
@@ -16,6 +19,7 @@ pub use markup::MarkupKind;
 pub use semantic_tokens::{
     SemanticToken, SemanticTokenModifier, SemanticTokenType, SemanticTokens, semantic_tokens,
 };
+pub use signature_help::{ParameterDetails, SignatureDetails, SignatureHelpInfo, signature_help};
 
 use ruff_db::files::{File, FileRange};
 use ruff_text_size::{Ranged, TextRange};