Simplify “**” handling using brace expansion (fixes #11110)

[gnodet]

This PR simplifies support for the ** wildcard and clarifies brace handling.

What changes

• Convert **/ to {**/,} after escaping special characters (including braces)
• Escape user-provided { and } in Maven-style patterns so braces are treated literally
• Keep brace alternation available when the user explicitly opts into glob: syntax
• Add focused tests for literal braces and explicit glob: alternation

Why

• Provides clear, POSIX-like behavior for ** (“zero or more directories”) without custom logic
• Aligns implementation with the documented rule that [ ] and { } are escaped in Maven-style patterns
• Ensures only the injected braces participate in expansion; user braces remain literals unless glob: is used

Behavior notes

• ** continues to mean “0+ directories”; **/ is normalized to {**/,} so both the current directory and subdirectories match
• Users who relied on unescaped braces acting as alternation in Maven-style patterns may see a behavior change; this corrects behavior to match the documentation
• With glob: prefix, brace alternation remains supported per NIO glob rules

Tests

• testLiteralBracesAreEscapedInMavenSyntax: verifies that files with literal braces match with Maven-style patterns
• testBraceAlternationOnlyWithExplicitGlob: verifies alternation works only with glob:

Fixes #11110

[desruisseaux]

Needs to be backported to the maven-4.0.x branch as this pull request also fixes the testWildcardMatchesAlsoZeroDirectory test case added by #11551. Waiting to see if #11551 is accepted in order to opportunistically include the two pull requests in a single backport.