lint rules for mixed indentation

[a-frantz]

We already have CommandSectionMixedIndentation but we need to rework it a bit.

• The SPEC has changed how mixed indentation is handled between v1.1 and v1.2 (and maybe from v1.0 to v1.1? Need to check) and the rules need to correctly handle this across different versions
• The existing rule only looks at command sections as they correctly deserve special treatment
  • But we also need to check for an entire document
  • IMO inside a command block mixed indentation is warning worthy, as it can impact parsing
  • Outside the command block we probably only want to emit a note
• Undecided on whether this should all be controlled by one rule or if we should divide and conquer