validate costume names and sprite names

[faretek1]

#333 #332 #331

ensure that no sprites or costumes are given the same names as special scratch sentinel values, e.g.

sprites: _mouse_, _myself_, _edge_

costumes: previous backdrop previous costume, next costume <- possibly these should just be warnings, but they should be given

these arent all sentinel values though, and im not sure if previous backdrop or next backdrop are valid. it needs to be checked

[linear]

GS-284

[faretek1]

i.e. a sprite file called _mouse_.gs should be invalid. however if it is just used in %include then it should be fine ofc