Prior to this commit, the `PathPattern` and `PathPatternParser` would
allow multiple-segments matching and capturing with the following:
* "/files/**" (matching 0-N segments until the end)
* "/files/{*path}" (matching 0-N segments until the end and capturing
the value as the "path" variable)
This would be only allowed as the last path element in the pattern and
the parser would reject other combinations.
This commit expands the support and allows multiple segments matching at
the beginning of the path:
* "/**/index.html" (matching 0-N segments from the start)
* "/{*path}/index.html" (matching 0-N segments until the end and capturing
the value as the "path" variable)
This does come with additional restrictions:
1. "/files/**/file.txt" and "/files/{*path}/file.txt" are invalid,
as multiple segment matching is not allowed in the middle of the
pattern.
2. "/{*path}/files/**" is not allowed, as a single "{*path}" or "/**"
element is allowed in a pattern
3. "/{*path}/{folder}/file.txt" "/**/{folder:[a-z]+}/file.txt" are
invalid because only a literal pattern is allowed right after
multiple segments path elements.
Closes gh-35213
Brian Clozel
d06255214e