Implement completions for union case field names in patterns

[kerams]

Unfortunately, VS does its own sorting, so when these items are not the only thing on the list, they might be buried deep.

[kerams]

#15754 needs to go first.

[kerams]

I had to fix the range of SynPat.LongIndent when it takes named pat pairs and there's a redundant semicolon at the end - the range stops at the semicolon instead of the rparen. The parsing code around atomicPatsOrNamePatPairs is ever so slightly simpler now.

cc @auduchinok

[kerams]

The CI is gunted. Otherwise ready.

[vzarytovskii]

@dotnet/fsharp-team-msft can I get a second approval pls?