Read-only/Write-only extern properties, Read-only vector properties

[PhoenixWhitefire]

Implements the read and write property attributes for external type definitions, which the embedded vector type now makes use of:

Before:

declare extern type vector with
    x: number
    y: number
    z: number
end

After:

declare extern type vector with
    read x: number
    read y: number
    read z: number
end

The following code now creates type-errors in the expected places:

--!strict
local function increment(v: vector)
    v.x += 1        -- TE
    v.x -= 1        -- TE
    v.y *= 1        -- TE
    v.z /= 1        -- TE
    print(v.x)      -- No TE
    print(v.x > 5)  -- No TE
    v.x = 15        -- TE
end

increment(vector.create(1, 2, 3))

This PR also supports different read/write types:

-- Definition
declare extern type Foo with
    read Bar: number
    write Bar: string
end

-- Script
local f: Foo
local b: number = f.Bar
f.Bar = "Hello, World!"

Additionally,

• Adds two new tests for the implemented syntax
• Adds the LuauLValueCompoundAssignmentVisitLhs, LuauExternReadWriteAttributes and LuauTypeCheckerVectorReadOnly flags

Closes #2062

[Stupid-Noob-Scripter]

I don't know what any of these means but I think they're pretty cool
this deserves 6-7 stars

[andyfriesen]

Thanks for the patch!

This all looks fine. We just need you to flag it before we can merge it.

[PhoenixWhitefire]

The flags LuauExternReadWriteAttributes and LuauTypeCheckerVectorReadOnly have been added
LuauTypeCheckerVectorLerp2 has been removed to prevent there being too many different combinations of the embedded vector definition
LuauVectorLerp no longer has any effect on the VM outside of allowing fastcalls. It felt better to do this than patch the failing test complaining about it not existing despite being defined in the type system due to the above flag removal (maybe the rest of the lerp rollout flags could also be removed, but that felt even further outside the scope of this PR and I didn't want to bring too many different areas into this)

[PhoenixWhitefire]

Added the flag LuauLValueCompoundAssignmentVisitLhs because I need to fix that bug in #2137 as well

[PhoenixWhitefire]

LuauTypeCheckerVectorLerp2 was clipped in 703 and I only noticed today 🙃