Crash during signature help

[TwitchBronBron]

brighterscript version: 0.69.6

It happens when you are defining a function, and add a comma at the end without having a closing parentheses:

sub test(p1 = invalid, <-- adding this comma causes the crash
  result = 1
end sub

We need to:

• add unit tests to verify we can reproduce the crash,
• then fix the issue by:
  • prevent throwing an exception when this happens
  • ideally improve the AST parsing to still produce mostly-valid AST rather than failing the whole parse.