Allow "unsafe" refactoring code actions (with warning)

[webspam]

Features

• Standardised and improved side-effect detection for code actions (extract variable, inline variable, and split/join local var declarations)
• When detected, the action is still offered but marked "(unsafe)", allowing an informed decision

Refactoring

• Consolidates logic into the BodyModel abstraction
• Completely rewrites and consolidates large tracts of disparate code action logic
• Body block is now compiled once per event instead of partially-compiled for every action
• Large clean up; deduplicates several functions
• Merges three extract action handlers into one shared handler