Add jsonassert receiver API

[nfisher]

Motivation

• Provide typed assertion wrappers so JSON assertions can be invoked via a constructor/receiver API (e.g. jsonassert.String(actual).EqualTo(expected)) for clearer chaining and consistency with other hammy packages.
• Preserve existing semantic behavior and internal helpers while making the public package-level functions delegate to the new receiver methods.

Description

• Add wrapper structs StringAssert, BytesAssert, and ReaderAssert and constructors String, Bytes, and Reader in hammy/jsonassert/jsonassert.go to hold the actual value.
• Implement receiver methods such as (*StringAssert) EqualTo, (*StringAssert) LinesEqualToWithOptions, (*BytesAssert) EqualToWithOptions, (*ReaderAssert) EqualTo, plus IsValid, Contains, PathExists, PathMissing, PathEqual, and array helpers, and delegate the original package-level functions to these methods.
• Keep internal helper functions like readJSON, linesContain, applyOptions, deleteJSONPath, sortJSONArrayAtPath, containsJSON, path parsing, and number normalization unexported and unchanged as implementation details.
• Update tests and examples to use the constructor/receiver API and adjust README snippets to show jsonassert.String(...), jsonassert.Bytes(...), and jsonassert.Reader(...) usages.

Testing

• Ran go test ./hammy/jsonassert and the package tests passed successfully.
• Ran the full test suite with go test ./... and all packages (including updated examples/tests) passed successfully.

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Codex Task