Fix custom comment continuation for multiple follow-up lines

[bobbrow]

Fixes: #8998

Cherry-picks the original change from the release branch. It should have gone to main first.
Also adds a unit test.

[sean-mcmanus]

@bobbrow There's a bug whenever one begin pattern is a prefix of another, e.g. if // and /// or /* and `/*!" are used, it causes the "continue" of the non-matching, shorter pattern to be used.

This works correctly,

    "C_Cpp.commentContinuationPatterns": [
        {
            "begin": "/*!",
            "continue": " ! "
        }
    ],

but adding a "/*" breaks it (uses * as the continuation instead):

    "C_Cpp.commentContinuationPatterns": [
        "/*",
        {
            "begin": "/*!",
            "continue": " ! "
        }
    ],

similarly, "//" is used as the continue for "///".

I could file a new bug for that (it's a pre-existing bug and not a regression from this PR).

UPDATE: I moved it to #14081 (with a fix at #14082)