Hide implicit self from __call signature help

[Companion]

Problem

Signature help for a table invoked through its __call metamethod includes the first metamethod parameter:

local module = setmetatable({}, {
    __call = function(self, value: string)
    end,
})

module()

Luau supplies self implicitly, so callers cannot provide it. However, signature help currently displays:

function module(self: any, value: string): ()

and reports both values as caller provided parameters.

Change

Build __call signature help from a copy of the metamethod type with its implicit first argument removed. The corrected signature is:

function module(value: string): ()

This removes self from both the displayed label and the LSP parameter list while preserving parameter offsets, active parameter tracking and documentation indices for the remaining arguments.

Ordinary function calls and existing method call (:) behaviour are unchanged. The separate method call label behaviour tracked by #1250 remains outside the scope of this change.

Fixes #1597.

Testing

Added focused signature help tests covering:

• Removing the implicit argument from the label and parameter list
• __call metamethods containing only self
• Multiple explicit parameters and their label offsets
• Active parameter tracking
• Variadic parameters
• Parameter documentation indices
• Mixed named and unnamed arguments
• Unchanged ordinary function behaviour

The new tests were verified to be load bearing by temporarily disabling the trimming and documentation offset behaviour.