Hide lines prefixed by dollar sign ($) in Luau code blocks

[itsmath]

This PR introduces a feature that hides comment lines inside ```luau code blocks when they are prefixed by a dollar sign ($).

Initially, I considered # for this purpose since it has been used for this in other languages, like Rust. But I opted for $ because:

1. # is valid Luau syntax in certain cases:

   local a =
   #b

   While this is rare, it could lead to unintended behavior.

2. $ currently has no use in Luau and is unlikely to be used in the future, especially if RFC: Reserve the dollar sign token to be forever unused luau-lang/rfcs#44 is accepted.

The lines will only be hidden if they are inside ```luau code blocks. This means you can still have a PHP code block, for example, and it'll work fine.

$this_wont_be_hidden = "because it isn't in ```luau";

If you need to start a line with $, you can use a regular ``` code block, which is still interpreted as a Luau code block by editors, but does not have this behavior.

Use Case

If you're implementing a documentation testing tool, or really any tool that needs to inspect the Luau code inside comments, this feature is very useful.

--- Adds `a` and `b`.
---
--- # Example
---
--- ```luau
--- $ local utils = require("@utils")
--- assert(utils.add(77, 33) == 110, "77 + 33 must be equal to 110")
--- ```
function utils.add(a: number, b: number): number
    return a + b
end

---

I probably should've started with a GitHub issue first to see if this is even something we want to do, but I was already writing this implementation for my own use, so might as well send a PR.