Implement type negation syntax

[PhoenixWhitefire]

Implements syntax as per negation-types RFC (luau-lang/rfcs#29)

• Adds flags LuauTypeNegationSyntax, LuauTypeNegationSupport
• Adds 1 parser test and 13 inference tests (some copied and changed from using a type function)
• Adds InvalidNegation error

local x: ~number = 5
-- TypeError: Expected this to be '~number', but got 'number'; 
-- the negation `~number`, and `number` is not a subtype of `~number`

local y: ~{ x: number }
-- TypeError: It is not possible to negate the type `{ x: number }` as, being a table type, it is non-testable.

[PhoenixWhitefire]

Just need to convert it to use a negate internal type function and should be good

[PhoenixWhitefire]

Have updated it to use an internal negate type function as per RFC

[PhoenixWhitefire]

A PR has been made by Alex, so I will be closing this under the presumption that their implementation will be better.

#2242

[PhoenixWhitefire]

stole more code from alex

[PhoenixWhitefire]

draft until #2543 is merged