Add new flags to CI

[Hiviexd]

Currently, it's pretty easy to improperly write new flags as ::{FLAG=US}:: when in reality, it's missing spaces and should be ::{ FLAG=US }:: (happened with #8134)

A CI check would do wonders in not letting this error pass to production.

[TicClick]

I'll reuse this issue to write down another thought from clayton (via)

[9:58 PM] clayton: all i can think of is forcing every custom span (the :: my content here :: thing) to be a flag since thats all we use em for
[9:58 PM] clayton: if u fail to make the span at all though it wouldnt help