Improve jsonata legacy mode detection regex

[knolleary]

Fixes #3290

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

The current regex used to spot if msg is used in a JSONata expression has been updated to ignore case where it appears as .msg. For example, if msg.payload had a property called msg - msg.payload.msg.

[coveralls]

Coverage increased (+0.03%) to 67.438% when pulling 0cf9b5f on jsonata-compat-mode-fix into 6d2793c on master.