Support wildcard path elements at the start of path patterns

[bclozel]

While AntPathMatcher usage in MVC applications has been deprecated for a long time at this point, we recently received additional feedback about migration efforts.

PathPattern currently only allows path elements that match multiple path segments at the end of a path pattern; for example:

1. /spring-framework/file/** matches /spring-framework/file/some/file.txt
2. /spring-framework/file/{*filename} matches /spring-framework/file/some/file.txt and yields filename=/some/file.txt
3. /**/file/index.html is not allowed, pattern compilation fails
4. /{*project}/file/index.html is not allowed, pattern compilation fails
5. /spring-framework/**/index.html is not allowed, pattern compilation fails
6. /spring-framework/{*folders}/index.html is not allowed, pattern compilation fails

We would like to improve this situation by allowing 3) and 4). Cases 5) and 6) have been requested in the past (see #22847) but they will still be disallowed as the matching process is hard to define properly and creates undesired behavior.