Support unicode escape sequences in tagger scripts

Allow unicode escape sequences, e.g. \u6e56, in tagger script.

Rationale:

• This would allow explicitly inserting special characters
• Makes it easier to define some characters the user cannot write
• We could make invisible control characters visible (see PICARD-2200)

For the syntax use \uXXXX, where XXXX is a Unicode code point in hexadecimal. This is similar to how Python does it.

This is an extension to the syntax, but old scripts stay compatuble. \u is illegal in older versions.