* This enables full string lexical parsing of some rules previously not fully compliant, including:
* refine
* user-augment
* if-feature
* Also solves previous tokenization issues
* Previous implementation did not handle nested if-feature expressions
* As part of fixing: [YANG if-feature does not support nested boolean expression](https://github.com/clicon/clixon/issues/341)
* Added new yacc/lex parser for if-feature-expr string