Field type migration

[Procyonae]

Summary

None

Purpose of change

Closes #73967
Migration good

Describe the solution

Adds basic JSON field type migration
Adds a test to check it works
Adds it to the docs

Removes reading ancient legacy ints that corresponded to field types, if it tries to read a legacy int it silently skips that field types members and then continues normally.
Doesn't remove the legacy functions etc, will clean up in a separate PR.

Describe alternatives you've considered

Not letting Erk get away with renaming ids just because they don't like them
Nesting the ter/furn and field migration checks inside their respective non migration checks

Testing

Compiles and loads, tests compile, existing field submap test and new migration one both pass

Additional context

[github-actions]

Spell checker encountered unrecognized words in the in-game text added in this pull request. See below for details.

Click to expand

• Furniture amd terrain migrations

This alert is automatically generated. You can simply disregard if this is inaccurate, or (optionally) you can also add the new words to tools/spell_checker/dictionary.txt so they will not trigger an alert next time.

Hints for adding a new word to the dictionary

• If the word is normally in all lowercase, such as the noun word or the verb does, add it in its lower-case form; if the word is a proper noun, such as the surname George, add it in its initial-caps form; if the word is an acronym or has special letter case, such as the acronym CDDA or the unit mW, add it by preserving the case of all the letters. A word in the dictionary will also match its initial-caps form (if the word is in all lowercase) and all-uppercase form, so a word should be added to the dictionary in its normal letter case even if used in a different letter case in a sentence.
• For a word to be added to the dictionary, it should either be a real, properly-spelled modern American English word, a foreign loan word (including romanized foreign names), or a foreign or made-up word that is used consistently and commonly enough in the game. Intentional misspelling (including eye dialect) of a word should not be added unless it has become a common terminology in the game, because while someone may have a legitimate use for it, another person may spell it that way accidentally.