Add "no dirty" core rules

[jedrichards]

"No dirty" rules would check for layer styles that have diverged from their associated shared style.

Two new rules could implement this behaviour:

• text-styles-no-dirty
• layer-styles-no-dirty

Inspiration for these rules comes from this community plugin https://github.com/KevinGutowski/FindDirtyStyles

Approach

I haven't thought through the approach fully yet but it's likely to involve:

• For each layer/text style in the document
  • If it has an associated shared style
    • Compare the layer style with the shared style, and if they're different raise a violation (when comparing styles a possible approach is to compare their hash like we're already doing here)