[FEATURE] Export keywords, readability, punctuation as `Seq[String]`s

[AlexWeinstein92]

Is your feature already requested in another issue?

No

Is your feature a defect?

From the perspective of riddl-idea-plugin, yes

Is your feature request related to a problem? Please describe.

Manual updating of keywords in plugin

Describe the solution you'd like

A def on each of com.ossuminc.riddl.language.parsing.{Keyword, Readibility, Punctuation} that returns all its vals in a Seq`

Readability.ByAs need not be included in the Seq[String]s, as it is already defined individually for by and as

Describe the alternatives you've considered

None