Add useful factory methods to `PathPattern`

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`PathPattern` is introduced in #651 to create complex path patterns easily.
We will provide more common patterns as factory methods.
```java
// *.json
PathPattern.endsWith("json");
// /**/foo.txt
PathPattern.endsWith("foo.txt");

// /foo/bar/**
PathPattern.under("/foo/bar"); // or
PathPattern.startsWith("/foo/bar");

// Compose the existing path patterns
// /foo/bar/**/*.json
PathPattern.startsWith("/foo/bar").endsWith("json"); // or
PathPattern.builder()
.startsWith("/foo/bar")
.endsWith("json")
.build();

// *.json and /foo/bar/**
PathPattern.of(PatternPattern.of("json"), PathPattern.startsWith("/foo/bar"));
```

Locate the PathPattern implementation introduced in #651 and inspect its existing pattern-composition API. Add the requested factory methods and composition forms described in this issue, then verify that the listed path examples produce the intended matches; no tests or file paths are named in the issue.

Written by the indexing model from the issue text.