Fix 'TODO: Add support for undefined escape sequence and Non-printing characters' issue in sifter\grammar\lexer.py

[manfred-kaiser]

Add support for:

• An undefined escape sequence (such as "\a" in a context where "a" has no special meaning) is interpreted as if there were no backslash (in this case, "\a" is just "a"), though that may be changed by extensions.
• Non-printing characters such as tabs, CRLF, and control characters are permitted in quoted strings. Quoted strings MAY span multiple lines. An unencoded NUL (US-ASCII 0) is not allowed in strings.

CodeFactor found an issue: TODO: Add support for:

It's currently on:
sifter\grammar\lexer.py:77
Commit 6e6601e