Add yamlfmt configuration schema

[drode1]

Summary

Adds the yamlfmt configuration JSON Schema for yamlfmt YAML formatter configuration files.

yamlfmt is an extensible command-line tool and library for formatting YAML files. The schema validates yamlfmt configuration files covering:

• Command settings - Line endings, path matching mode, include/exclude paths, regex-based exclusion, extension filters, continue-on-error behavior, output format
• Path discovery - Known config filenames, gitignore-based excludes, custom gitignore path, doublestar and gitignore matching modes
• Formatter selection - basic and kyaml formatter types
• Basic formatter options - Indentation, document start marker, line break retention, anchor handling, max line length, folded scalar handling
• Formatting behavior - Array indentation, merge tag handling, line comment padding, trailing whitespace trimming, EOF newline, directive stripping
• Style controls - Array style forcing, quote style forcing, root array indentation, alias-key correction control

File matches

.yamlfmt
yamlfmt.yml
yamlfmt.yaml
.yamlfmt.yaml
.yamlfmt.yml

Schema source

Documentation
Additional docs:

• one
• two

Tests

• basic-config.yaml - Comprehensive basic formatter configuration with command-level and formatter-level options
• implicit-basic-config.yaml - Formatter block without explicit type to cover default basic formatter behavior
• kyaml-config.yaml - kyaml formatter configuration
• invalid-force-array-style.yaml - Rejects unsupported force_array_style values
• invalid-force-quote-style.yaml - Rejects unsupported force_quote_style values
• invalid-line-ending.yaml - Rejects unsupported line_ending values
• invalid-match-type.yaml - Rejects unsupported match_type values
• invalid-output-format.yaml - Rejects unsupported output_format values

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.