consider deduplicating more content

[lynxlynxlynx]

use includes or collections where appropriate (identical opcodes, actions, ...) and mark them as such for the user.

• opcodes
• actions
• 2das mark 2da that are equal #19

[lynxlynxlynx]

work on merging/deduping opcodes is under way (see _opcodes/ dir for format).

[lynxlynxlynx]

maybe add links to other versions of the same item near the title, so cross-engine comparisons are easier.
And check if you can use a shared layout for all the opcode listings again.

[lynxlynxlynx]

main opcode work is done, including code share. Other stuff encountered is now listed as separate issues. Also linking of other versions was coded in. Splitting the remaining action dedupe into a separate issue.