Support for unicode escape sequences in String literals

E.g. ```'P\u0065ter'``` should be 'Peter'

FHIR R4 Exclusions


**Details**

```Expected: true but got: false ==> expected: <true> but was: <false>```
- ```Patient.name.given.first() = 'P\u0065ter' [** testLiteralUnicode]```