Adds JSON schema support for rumdl, a fast Markdown linter and formatter
written in Rust.

Schema Details:
- Name: rumdl
- File matches: .rumdl.toml, rumdl.toml
- Schema URL: https://raw.githubusercontent.com/rvben/rumdl/main/rumdl.schema.json
- Project URL: https://github.com/rvben/rumdl

What is rumdl?
rumdl is a fast, configurable Markdown linter that implements 54+ rules
for consistent Markdown formatting. The schema enables IDE autocomplete,
validation, and documentation for configuration files.

Schema Features:
- Automatically generated from Rust types using schemars
- Validates global configuration options (disable/enable rules, file patterns)
- Validates per-file-ignores configuration
- Validates rule-specific configuration
- Supports multiple markdown flavors (standard, mkdocs)

Testing:
- Added test files in src/test/rumdl/
- Schema validated against example configurations