Parser: add RegisterSingleUseKeyword

[IhateTrains]

In lots of cases where we use the parser, each keyword is expected to appear only once.

Below is an example of a CK3 dynasty definition. prefix, name, culture, motto are all expected to appear only once.

695 = {
	prefix = "dynnp_ua"
	name = "dynn_Briain"
	culture = "irish"
	motto = dynn_Briain_motto
}

By unregistering the keywords after they are matched, we can improve the performance of the parser. The first token will have to be matched against 4 keywords, but the second against 3, third against 2 and the last one against just 1 keyword.

Should probably be benchmarked, though.