feat: Implement textDocument/signatureHelp for function/action signatures

[jade-codes]

Summary

Add support for textDocument/signatureHelp LSP capability.

Use Case

When typing a function/action call, users should see parameter information:

• Parameter names and types
• Current parameter position
• Documentation for each parameter

Example

action def doSomething(in x: Real, in y: String) { }

action usage {
  perform doSomething(|)  // <-- signatureHelp shows: doSomething(in x: Real, in y: String)
}

Implementation Notes

• Trigger characters: ( and ,
• Similar to hover, but specifically for callable symbols
• Return SignatureHelp with SignatureInformation and ParameterInformation

References

• LSP Spec: textDocument/signatureHelp