Remove extern ter ids

[Procyonae]

Summary

None

Purpose of change

Remove everything but t_null from the ancient (Pre 0.A) int_id<ter_t> list from mapdata.h/.cpp
Extern evil idk

Describe the solution

Changes references to the extern list for static const string_id<ter_t> definitions at the top of each file that uses them in line with other ids
Uses ter_str_id::NULL_id() instead for t_null

Left a few instances of t_null that were being awkward, if someone more technically adept wants to attack them here's the list

mapdata.cpp L64
mapgen.cpp L854 L4376 L5144 L7690
regional_settings.h L100 L283
submap.h L318

Changes a few excessive checks to use unordered_set and find instead

Describe alternatives you've considered

I were going to just remove the ones that aren't referenced anymore but anothersimulacrum and kevingranade both advocated for changing to using string_id<ter_t>s per file

Keeping the ids as t_ as opposed to the currently used ter_t_ for string ids and instead changing existing ter_t_ to t_ but that's probably at least just as intrusive and other types of static id use the existing format

Not causing myself merge conflicts

Testing

Game compiles and loads fine

I wouldn't be surprised if there's some random place the int_id -> string_id causes an issue that I haven't noticed and the tests don't catch but hopefully it's nothing major

Additional context

There's alot of stuff that can be replaced with flag/ter_furn_transforms instead of needing to use hardcoded ids, might attack some in future PRs

[nornagon]

You're a saint