[C#] use standard unclosed-string behavior

[keith-hall]

This PR changes the unclosed string behavior in the C# syntax to match that of the other syntaxes, like Python, which scopes the new line character as invalid, which is less jarring than having the whole string appear as invalid.

[michaelblyons]

Great! I do not think this should get in the way of merging, but there are some rules about newlines in interpolation that we can work on at some point.

string good1 = $"bar { baz }";
string good2 = $@"bar { baz }";
string good3 = $@"bar {
    baz }";
string good4 = $@"bar {
    baz
    }";
string good5 = $@"bar
                { baz }";

string bad1 = $"bar {
    baz }"; // Interpolation must be single line without $@"..."

This would be pretty easy if you can with_prototype into an include instruction, but I suspect it's more likely that the string_interpolation construct will have to be broken up.

[keith-hall]

I've improved it a tiny bit, but something like string unclosed_interpolation = $"inner {new[] { 12, 24 won't be detected as invalid atm. I can't use with_prototype here because we hit this issue: sublimehq/sublime_text#1288

[michaelblyons]

Excellent, this is much more elegant than I had expected. =)

[wbond]

Thanks!