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

crates/nu-cli/src/completions/completer.rs

@@ -325,8 +325,8 @@ impl NuCompleter {
         // unfinished argument completion for commands
         match &element_expression.expr {
             Expr::Call(call) => {
-                // TODO: the argument to complete won't necessarily be the last one in the future
-                // for lsp completion, we won't trim the text,
+                // NOTE: the argument to complete is not necessarily the last one
+                // for lsp completion, we don't trim the text,
                 // so that `def`s after pos can be completed
                 for arg in call.arguments.iter() {
                     let span = arg.span();