Allow defining the size of arrays

[C3pa]

Feature request
I'd want a way to define the size of arrays. I propose the following:

---@type string[3]
local arr = {}

[C3pa]

So, to provide more feedback, the desired behavior would be triggering a warning when accessing elements out-of-bounds. Here is an example:

---@class point
---@field x number
---@field y number
---@field z number

---@class rect
---@field id string
---@field points point[4]

local rect = require("rect") -- the implementation details are irrelevant

---@type rect
local myRectangle = rect.new()

local pointA = myRectangle.points[6] -- Out-of-bounds array access warning triggered here

[C3pa]

In addition that might be used to infer the value of the table length (#) operator. Not sure if that is a good idea though.

---@type string[3]
local arr = {}
local n = #arr 
-- n = 3 from now on

[sumneko]

According to my experience, in most case the indexes are calculated instead of literal when out-of-bounds.
However, because it is not difficult to implement, I can implement it as part of the type-check.

[C3pa]

I came up with a new usage example that this feature enables. Let's consider the following code:

---@return integer[3] RGBColor
local function getColor() 
	return {
		255, -- red
		255, -- green
		255  -- blue
	}
end

local red, green, blue = unpack(getColor())
-- red, green and blue's type is integer now.

local red, green, blue, alpha = unpack(getColor())
-- red, green and blue's type is integer, alpha is inferred as nil.

Edit: inferring of types from the unpack call is actually requested in issue #1353.

[Groctel]

When this is implemented, I think that the server should check that the typed table is created with N elements and appending to it should report a warning.