Add --validate flow with per-entry no_expand

[MikeGarde]

Enable standalone validation and expansion control across files

• Add -V/--validate CLI flag to run env-file validation as a separate flow and disallow conflicting options
• Introduce per-entry no_expand flag on EnvValue; default remains false and single-quoted values are treated as literals when set
• Refactor parsing to collect all formatting issues (whitespace around = and duplicates) and return a single EnvParseError with the first offending line and a combined message; apply no_expand for single-quoted values
• Centralize path resolution in resolve_env_path (defaults, absolute resolution, existence checks); main uses it and prints <path> is valid on success
• Update error output to print only the error message and remove ad-hoc path handling in run paths
• Update README: align badge to Rust branding and add a “Validating a File” section with an example (dotenv --validate --file .env.example)
• Add tests/fixtures for validation: space.env and singleQuote.env, test valid/invalid/missing cases, exclusivity with other options, and whitespace/duplicate-key diagnostics
• Improve error messaging for unterminated quotes and extend tests for quote-sensitive expansion and end-to-end fixture validation

[codecov]

Codecov Report

❌ Patch coverage is 95.89041% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/env_object.rs	85.71%	1 Missing ⚠️
src/env_parser.rs	96.87%	1 Missing ⚠️
src/main.rs	97.05%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/env_object.rs	80.68% <85.71%> (+0.19%)	⬆️
src/env_parser.rs	92.39% <96.87%> (+0.67%)	⬆️
src/main.rs	72.97% <97.05%> (+2.51%)	⬆️

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