Implement read-only/write-only table indexer syntax

[PhoenixWhitefire]

Additionally adds the LuauReadWriteOnlyIndexers and LuauLValueCompoundAssignmentVisitLhs flag and 6 new tests

The tests are casting string indexing to :: string explicitly because there seems to be an unrelated bug where string singletons are not checked properly

Defined in RFC https://rfcs.luau.org/syntax-property-access-modifiers.html, https://rfcs.luau.org/property-writeonly.html, https://rfcs.luau.org/property-readonly.html

The LuauLValueCompoundAssignmentVisitLhs was added to prevent a merge conflict with #2071, which implements the same fix.

-- These now work:
type T = { read number }
type U = { write [vector]: string }

[PhoenixWhitefire]

Not finished yet:

--!strict

function _foo(x: { read number })
	local y: { number } = x
	y[1] = 5 -- No Type Error
end

EDIT: Fixed, and improved error messages

[PhoenixWhitefire]

[PhoenixWhitefire]

index?