Add syntax check for single/double quotes and square brackets in splitter

Ion panicked with invalid syntax on square brackets and quotes, see :

• #990
• #942
• #914

I did not really know were to add additional checks so I added them

in the statement splitter and it seems to work. I thought the splitter

was responsible to validate the command syntax but it looks like checks

are spread all across the parsing process which make sense when dealing

with multiple stage parsing(expansions etc..)

Please note that some checks are performed multiples times along the

parsing process, maybe this could be improved but I do not feel like

doing it since Ion's grammar is still a bit obscure to me.

I also added tests to cover all the splitter code and moved skip as local variable.