feat: add register_sym_simp command and SymSimpVariant infrastructure

[leodemoura]

This PR adds the register_sym_simp command for declaring named Sym.simp
variants with pre/post simproc chains and optional config overrides.

register_sym_simp myVariant where
  pre  := telescope
  post := ground >> rewrite [thm1, thm2] with self
  maxSteps := 50000

• SymSimpVariant structure storing pre?/post? syntax (elaborated at use
  time in GrindTacticM) and Config overrides
• SimpleScopedEnvExtension for persistent cross-module variant storage
• register_sym_simp command syntax with sym_simp_field category
• Command elaborator with syntax quotation matching and duplicate field detection
• getSymSimpVariant? lookup function
• deriving Inhabited on Simp.Config for extension support