partially implement type:setreadindexer

[nnullcolumn]

previously, setreadindexer would error with type.setreadindexer: luau does not yet support separate read/write types for indexers.

now that read-only indexers are implemented, this PR suggests a partial implementation locked behind a new flag LuauUdtfTypeSetReadIndexerMethod and an old one LuauTypeFunctionTableIndexerIsReadOnly for serialization/deserialization. It will error if the table already has a read+write indexer, but otherwise allows the operation. Although partial implementation isn't ideal, some trivially-fixed-once-separate-read-write-indexers-are-implemented dragons are okay as this gets behavior into users' hands sooner and UDTFs are inherently forgiving towards mistakes. As usual, this PR / implementation (although small) is 100% human authored.

edit: the existing workaround people are likely to use, provided LuauTypeFunctionTableIndexerIsReadOnly is enabled:

type tbl_with_read_only_indexer<key, value> = { read [key]: value }

type function setreadindexer(tbl: type, index: type, value: type): type
	const new: type = tbl_with_read_only_indexer(index, value)
	const mt = tbl:metatable()
	if mt then
		new:setmetatable(mt)
	end
	for key, prop in tbl:properties() do
		const read, write = prop.read, prop.write
		if read and write and read == write then
			new:setproperty(key, read)
		else
			if read then
				new:setreadproperty(key, read)
			end
			if write then
				new:setwriteproperty(key, write)
			end
		end
	end
	return new
end

[nnullcolumn]

whoops broke my own tests by changing an err msg :p