DEV 1676: Add maps helper functions

[robbles]

Adds a Validate method to Metadata for checking and generating helpful errors when the number of players or map size is invalid, and a bunch of accessor methods to Editor for checking what's on the board already.

The refactoring of existing maps to use these will be in a follow-up ticket.

[codecov-commenter]

Codecov Report

Merging #111 (29af042) into main (c5810d8) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
- Coverage   70.47%   70.36%   -0.12%     
==========================================
  Files          33       33              
  Lines        2415     2470      +55     
==========================================
+ Hits         1702     1738      +36     
- Misses        641      664      +23     
+ Partials       72       68       -4     

Impacted Files	Coverage Δ
maps/castle_wall.go	68.57% <ø> (ø)
maps/game_map.go	100.00% <100.00%> (+5.71%)	⬆️
maps/hazard_pits.go	88.23% <100.00%> (+3.32%)	⬆️
maps/arcade_maze.go	53.22% <0.00%> (-33.88%)	⬇️
maps/royale.go	66.66% <0.00%> (-5.56%)	⬇️

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