Fix expression visitor to make escape work in nested entity definition

[feich-ms]

Closes #1107. Fix expression visitor logic to make escape work in nested entity definition, typical cases are:

# BoldIntent
- {@Command={@Action={@BoldAction=bold}} \{{@ActionTargetPhrase=directions from seattle to portland}\}},
- {@Command={@Action={@BoldAction=emphasise from}} {@ActionTargetRange={@ActionTargetStart=@username {@ActionTargetSeparator=through} {@ActionTargetEnd==}}}

After the fix, the escape brackets in nested brackets will work and the equal sign as entity value will also work.