This PR fixes #72177

When autoclosing multi-character pairs, rather than just adding all of pair.close, it now only adds the part that hasn't already been autoclosed.
It does this by finding the longest pair that is contained in the current pair. It then checks if subAutoClosingPair.close is already present (it may not necessarily have just been autoclosed) for all selections, and if so only adds a substring of pair.close excluding subAutoClosingPair.close.

In order to get auto close working when the sub-pair is multiple characters, _isBeforeClosingBrace had to be updated to work with multi-character pairs.

And example of where this is the case is the F# pairs

"autoClosingPairs": [
...
["<@", "@>"],
["<@@", "@@>"],
...
]

In the process of updating _isBeforeClosingBrace, I changed it from excluding symmetric pairs to explicitely returning false for all potential starting braces.
I believe the intended purpose of the symmetric pair check was to allow autoclosing before close braces but to not get mixed up when the close brace might actually be an open brace (see #20801).
However, the previous implementation also allowed autoclosing before starting braces if the autoclose brace was also symmetric. This seems contrary to the method name so I'm assuming it was unintentional (or otherwise it should probably be more explicit);
this change required a couple of minor changes to some unit tests.