Correct escape sequence highlighting (no need for word boundary)

Doxygen source code comments have support for escape sequences:

Example: @% or % are escape sequences that represent the percent sign.

ACTUAL BEHAVIOUR:

In the sequence @%L1 everything is highlighted like an unknown command.

EXPECTED BEHAVIOUR:

In the sequence @%L1 only the substring @% is highlighted, and like a character.

NOTE:

This MR implements this by removing the required word boundary in the existing regular expression, and by defining a new style (different from the command style) for character escape sequences. (Though escape sequences have the same syntax as commands, their semantic is quite different, which makes an own style reasonable.)