Fix signature help in CEs and use better heuristics

[cartermp]

fixes #10808

Several things were actually wrong here, but this corrects everything as far as I can tell. In summary:

• Don't go hunting for method parameter help as a means to see if we should go down that path; instead base it off the trigger character and if the adjusted column (relative to the caret position) in source is also a character indicating we're inside a method call
• Account for the fact that a CE is itself an application, so when we check if we're in one, account for this case, lest we erroneously attribute the application of async { ...} as a relevant function application
• Add some testeronies

[KevinRansom]

nice