Fixes signature help edge case

[ncordon]

What

Fixes the case of getting the overlay with the information about the first argument for functions.

How

RETURN apoc.do.when( gets parsed as:

      statements
     /    |    \     
statement (    EOF
    /  \
RETURN  expression

rather than:

      statements
     /    |    \     
statement (    EOF
    /  \
RETURN  functionInvocation

so we need to treat that case differently because we cannot modify the relative priority of functionInvocation vs expression

RETURN apoc.do.when(x gets parsed correctly in contrast (when we've started the first argument), because the parser has enough information to recognize it is a function invocation.

When our final character is (, just go to the preceeding statement and traverse it to find whether it ends on a expression. If it does, that's our function name.

[OskarDamkjaer]

I worry a little bit about false positives with this approach (since expressions are so flexible), but I can't think of any compelling examples or any other solution barring looking into changing the error recovery so I think it makes sense to go ahead