Support string escaping

[spoenemann]

We should support string escaping in the grammar language:

• Add \ escaping to the STRING rule in langium-grammar.langium
• Process escaped characters in the convertString method of DefaultValueConverter
• Adapt the syntax highlighting config of the grammar language to support escaped characters

Discussed in #884

^{Originally posted by ballcoach12 January 26, 2023}
The following rule generates an error in the grammar editor:

terminal S_BYTE_CHAR_STR returns string:
 	'\'' S_BYTE_CHAR_VALUE* '\''
 ;

The second backslash causes the following error: Unexpected character value: ->\<-. Do I need to escape this character somehow?

[spoenemann]

See also #797, which is about including escaped characters in the generated syntax highlighting. Shall we try to detect whether the terminal rule supports escaped characters? Or add that to syntax highlighting without checking?