Escape sequences in string literals

[MeAmAnUsername]

Summary
Implement and document proper escape characters in string literals

Todo

• Find reference documentation for what characters exist in Java and what they mean
• Decide which escape sequences will exist
• Write tests
• Implement
• Update documentation

Reason
Compiler generates incorrect Java code when using an escape sequence which is not legal in Java.

Reproduction steps
Given the string literal "\c"
Expected: it compiles to "c" (with possibly a warning that \c does not do anything) or is an error
Actual: compiles to it compiles to "\c", which gives the following error in Java: "Illegal escape character in string literal"

Description
Currently it just allows everything, and directly passes it to Java (with exception of \$). This does not work because apparently java does not allow invalid escape sequences.
Should decide whether to allow invalid escape sequences (and give a warning that it does not do anything) or to make it an error (like Java).
Probably easiest to just copy the sequences from Java, plus \$

Implementation
Probably best to desugar to the actual character code, and then compile back to escape sequence when generating Java.
This means that the string used internally in the compiler and static semantics is always the literal string without escape sequences.