Add TOML parser and schema validator

[snelusha]

This pull request introduces a new Go TOML parsing library with schema validation support, comprehensive diagnostics, and an extensive test suite. The implementation leverages third-party libraries for TOML parsing and JSON schema validation, and provides user-friendly APIs for reading, querying, and validating TOML files.

Core TOML parsing and querying functionality:

• Added the initial implementation of the Toml struct and related types in tomlparser/toml.go, supporting reading TOML from files, streams, and strings, as well as querying values by dotted keys, extracting tables and arrays, and converting TOML data to Go structs. Diagnostics are recorded for parsing and mapping errors.

Schema validation support:

• Introduced a generic Schema interface and implementation in tomlparser/schema.go using the jsonschema/v6 library, allowing validation of TOML data against JSON schemas loaded from files, strings, or file handles.
• Added a Validator interface and implementation in tomlparser/validator.go to validate TOML documents against schemas and append diagnostics on validation errors.

Dependency management:

• Updated go.mod to add dependencies on github.com/BurntSushi/toml for TOML parsing and github.com/santhosh-tekuri/jsonschema/v6 for schema validation.

Testing and sample data:

• Added a comprehensive test suite in tomlparser/toml_test.go covering parsing, querying, diagnostics, and schema validation scenarios.
• Provided sample TOML files and JSON schemas for testing in the testdata directory, including valid, invalid, and missing-required-field examples. [1] [2] [3] [4] [5] [6]