feat(lsp): signature help (manually triggered) (#15233)

# Description

To check for missing parameters

<img width="417" alt="image"
src="https://github.com/user-attachments/assets/5e2a8356-5fd9-4d15-8ae6-08321f9d6e0b"
/>

# User-Facing Changes

For other languages, the help request can be triggered by the `(`
character of the function call.
Editors like nvim refuse to set the trigger character to space, and
space is probably way too common for that.

So this kind of request has to be triggered manually for now.
example of nvim config:

```lua
vim.api.nvim_create_autocmd("FileType", {
  pattern = "nu",
  callback = function(event)
    vim.bo[event.buf].commentstring = "# %s"
    vim.api.nvim_buf_set_keymap(event.buf, "i", "<C-f>", "", {
      callback = function()
        vim.lsp.buf.signature_help()
      end,
    })
  end,
})
```

# Tests + Formatting

+2

# After Submitting

tests/fixtures/lsp/hints/signature.nu

@@ -0,0 +1,17 @@
+str substring ..2 -g foo bar b
+(
+  # inside parenthesis
+  str substring
+  ..2 -b
+  foo
+  b
+
+)
+foo bar 1 2 3
+foo baz 1 2 3  
+def "foo baz" [
+    p1: int
+    p2: string,
+    p3?: int = 1 # doc
+] {}
+echo