[ShellScript] Improve highlighting accuracy of test expressions

[deathaxe]

According to the bash docs...

1. the test ... command doesn't support Perl like pattern matching via =~ operator.
2. supported features / allowed expressions differ between [ ... ] and [[ ... ]] operators.
3. highlighting would probably need to be distinguished between string/pattern matching and arithmetic comparisons
   • In case of presence of -eq or equivalent operator the test expression needs to be treated as arithmetic expression like within (( ... ))
   • otherwise tests compare strings or do pattern matching
   • or perform unary file tests

All different kinds of test statements share the same quite naive implementation at the moment, which causes some inaccuracies in some edge cases.