Implement support for Spanish

[alerque]

Closes #46, needs review from native speaker(s), see issue discussion.

[Copilot]

Pull Request Overview

This PR adds comprehensive Spanish language support to the decasify library, implementing titlecasing according to Real Academia Española (RAE) guidelines.

Key changes:

• Adds new Spanish language module with RAE-compliant titlecasing rules
• Implements lowercase, uppercase, and sentence case transformations for Spanish text
• Integrates Spanish locale and RAE style guide into the type system

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/es.rs	New Spanish language module implementing titlecase (RAE style), lowercase, uppercase, and sentencecase functions with Spanish-specific reserved words
src/lib.rs	Routes Spanish locale to new es module functions for all case transformations
src/types.rs	Adds ES locale variant, RealAcademiaEspanola style guide, and parsing support for Spanish locale identifiers (with/without accents)
tests/lib.rs	Comprehensive test coverage for Spanish transformations including trivia handling, article/preposition rules, and all case conversions
README.md	Documents Spanish support and RAE styling conventions with usage caveats
Makefile.am	Includes es.rs in build sources
plugin/decasify.vim	Adds 'es' to locale completions
plugin/decasify.lua	Adds 'es' to locale completions

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