Validate more names

[github-actions]

• _stage_, _myself_, _random_, _mouse_ for sprite names
• next costume, previous costume for costumes
• next backdrop, previous backdrop, random backdrop for backdrops

grazelang/grazelang/src/parser/cst.rs

Lines 2535 to 2539 in 4019287

	// TODO: Validate more names
	// - [ ] `_stage_`, `_myself_`, `_random_`, `_mouse_` for sprite names
	// - [ ] `next costume`, `previous costume` for costumes
	// - [ ] `next backdrop`, `previous backdrop`, `random backdrop` for backdrops
	// Issue: #89