dev/ci: rewrite computeRunType to use structured RunTypeMatchers

Created by: bobheadxi

Rewrite computeRunType to use structured RunTypeMatchers provided by each RunType to allow us to understand how a RunType is determined after the fact. See this in action in https://github.com/sourcegraph/sourcegraph/pull/30532. Demonstrative example:

https://github.com/sourcegraph/sourcegraph/blob/e562df81b1473c9f3b9b08a5cb16268e3bc21243/enterprise/dev/ci/internal/ci/runtype.go#L46-L54

Also introduces testing that iterates over RunTypes and ensures each has relevant properties defined, as well as tests covering some of the new and old functionality.