Fix ** glob pattern not matching files in root directory (Fixes #2355)

[amanmaurya92]

This fixes issue #2355 by updating the DoubleStarNode regex evaluation to treat a leading or trailing slash as optional when matching zero directories, and flattens AST nodes during list generation.

Fixes #2355.

Changes:

• Updated SequenceNode._toRegExp() in ast.dart to make the trailing slash following a DoubleStarNode optional ((?:[^]*/)?) when matching zero directories (e.g. allowing **/filename to match filename).
• Flattened the internal AST node structure in list_tree.dart's _join() method to correctly position adjacent SequenceNode elements so they are processed properly.
• Added tests in match_test.dart and list_test.dart to ensure **/foo matches foo and correctly evaluates when ** corresponds to 0 directories.

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Code Review

This pull request improves glob matching logic to correctly match files in the root directory when using double-star patterns (e.g., **/foo matching foo). It updates SequenceNode._toRegExp() to handle a DoubleStarNode followed by a slash literal, flattens nested SequenceNode instances in _join, and adds corresponding tests. The reviewer pointed out that the substring operation in SequenceNode._toRegExp() assumes the regex always ends with [^]*, which is fragile, and suggested adding a defensive check to ensure it ends with [^]* before performing the substring.

[mosuem]

I believe this is missing more edge test cases, for example: a/**/b matching a/b and a/x/b, or foo**/*.dart matching foo/a.dart but not foobar.dart.

[amanmaurya92]

The previous regex replacement was too loose.

I've updated the logic so the optional slash for ** only applies when it acts as a true directory boundary
(imagine: when it sits at the start of the string or directly follows a separator).

To ensure everything behaves exactly as expected, I've added the following tests now:

1. expect('a/b', contains(Glob('a/**/b'))); (Matches zero directories bounded by slashes)
2. expect('a/x/b', contains(Glob('a/**/b'))); (Matches one directory bounded by slashes)
3. expect('foo/a.dart', contains(Glob('foo**/*.dart'))); (Matches correctly when ** is attached to a literal)
4. expect('foobar.dart', isNot(contains(Glob('foo**/*.dart')))); (Ensures it doesn't overly aggressively match paths lacking a directory separator)