Non-recursive constructs recognition 

Partially covers #17. Implements recognition of backquoted version of command substitution, allowing it also inside double-quotes. Also implements recognition of parameter expansion for names and symbols not enclosed in braces: a) variable names as described in XBD 3.230, b) special parameters as described in XCU 2.5.2, and c) positional parameters as described in XCU 2.5.1. Note that only single-digit positional parameters are accepted, because multiple-digit positional parameters are required to be enclosed in braces. Recognition of ${...}, $(...), $((...)) constructs is postponed at this point as they need recursive algorithm, and cannot be covered solely by regular expressions supported by lex(1) utility. lex(1) utility internal table size constants set to values exceeding minimal values implementations are required to provide. This issue is to be covered separately.