Add string inspection and intentions

[hurricup]

Caused by #2218 Inspection should check string substitutions:

• [ ] Inspection should mark invalid \c sequence: unmappable, outside of 0-127, trailing, probably even useless.
• [ ] warn about useless \E
• [ ] mark ambiguous backref-octal constructs
• [ ] mark different escape sequences availablity in current target version
• [ ] mark useless escape
• [ ] we should probably provide quick-fixes to convert different substitutions