More parser error recovery, unlocking completions in more locations

[minestarks]

1. Callable signatures tolerate a lot more broken syntax now. This allows us to complete type parameters correctly in broken syntax that we commonly see when the user is in the middle of typing:

operation Test<'T>() : () => |

operation Test<'T>(x: |)

operation Test<'T>() : |

operation Test<'T>(x: Std.|) : Unit {}

2. Error recovery in notebook cells if a statement can't be parsed. Fixes Broken expression node at the end of a cell breaks the AST #1960 and allows completions to work in scenarios like this:

%%qsharp

function Foo() : Unit {}
let x = |  // expect Foo in the list

[swernli]

This is a really nice change, thanks! Of note for those who think about perf, I did run comparisons against main and saw no impact on parser performance.

[orpuente-MS]

The code and logic seem fine. Approved. These are amazing improvements. Thanks!