Type checking for templates

[0kku]

Unfortunately, @BobobUnicorn's PR to TS got closed, however the TS team said they're open to revisiting it at a later date. As such, we need to figure something else out to get type checking for templates. After exploring our options, it seems that just maintaining our own fork of TS seems like the least unreasonable approach for now. The goal with this move is to explore the viability of implementing an XML parser in TS types. The primary concern is performance. We're hoping to get the type checking performance with this approach to be acceptable, which would hopefully also convince the TS team that this is not a terrible thing to merge into the language officially.