Fix encoding bug

[sebinside]

There is still a bug that breaks Jplag if, e.g., german umlauts or other non-ASCII characters are used.

Releated to #7 and #77.

[tsaglam]

I fixed a few encoding issues by enforcing UTF-8 (#177 and #185), but these PRs concern the core and the Java 9+ frontend. Other language frontends might still use the system default charset when reading in files, thus leading to encoding issues.

[tsaglam]

Also, some language frontends use a grammar-based parser that explicitly limits identifiers to e.g. [a-z, A-Z, 0-9].

[tsaglam]

Closed, as this is also discussed in #746. This issue will be linked.