Summary

Infrastructure "terrain/furniture: code deduplication and improved copy-from support"

Purpose of change

Roughly the other half of #73211; copy-from terrain/furniture is a desirable feature

Closes #54702

Describe the solution

• TRANSPARENT flag inherits correctly
• flags support extend/delete
• connect_groups, connects_to and rotates_to support copy-from
• Removes legacy connects_to/rotates_to flag support (made possible by Remove reliance on legacy connection group flags #73415)
• Merges some duplicate code in ter_t::load and furn_t::load into map_data_common_t::load
• Makes map_X_bash_info and map_X_deconstruct_info optional, replaces bad "str_max > -1" checks

Sorry for the encroaching line count. A lot of the line "additions" are very minor, like '.' to '->'
This PR should not make any functional changes to gameplay.

Describe alternatives you've considered

What's left to be done (next on my to-do list):

• symbol,color supports copy-from
• remove LINE_OXOX and LINE_XOXO handling, replace it with better generic unicode character handling. The original PR attempted to do this, but it didn't work, so I didn't include it here.
• handle bash, etc. special inheritance handling

Once all that's done, attempt to implement abstract furniture/terrain; there's a ton of JSON bloat that can be removed.

Testing

Passed all tests, clang-tidy should be good

Made sure the following work as usual:

• deconstruct t_wall
• smash t_wall until broken (STR 40)
• repeat above with an identical copy-from child of t_wall
• blow up bomb
• burn down building
• test breaking down a tent
• using bolt cutters, hacksaw to cut a fence

I made five test walls and they all worked as expected, including connect groups and rotation:

• test wall 1 copied from t_wall and added "extend": { "flags": ["TRANSPARENT"] }
• test wall 2 copied test wall 1 and added "delete": { "flags": ["TRANSPARENT"] }
• test wall 3 copied from t_wall and added "delete": { "flags": ["FLAMMABLE"] }
• test wall 4 copied from test wall 2 and re-added "extend": { "flags": ["TRANSPARENT"] }
• test wall 5 copied from t_window (for a rotation check)

Additional context

Proof for #54702: removed flags from t_deck_coating_no_roof_r and it's still transparent

As with last time, thanks to @Procyonae for writing the original PR, I just cleaned it up