childeswordfreq 0.2.0

• Added phrase_counts() for surface phrase frequency extraction.
• Added disk caching with cwf_cache_enable() / cwf_cache_disable().
• Added full “Usage & Best Practices” vignette with usage guidelines.
• Improved DESCRIPTION with clearer database reference and URLs.
• Updated examples and documentation.
• Added initial testthat tests for word_counts() and phrase_counts().
• Minor stability and messaging improvements.