This commit introduces several improvements to the file checking process and URI handling:
- Extract file checking logic into separate `Checker` structs (`FileChecker`, `WebsiteChecker`, `MailChecker`)
- Improve handling of relative and absolute file paths
- Enhance URI parsing and creation from file paths
- Refactor `create_request` function for better clarity and error handling
These changes provide better support for resolving relative links, handling different base URLs, and working with file paths.
Fixes https://github.com/lycheeverse/lychee/issues/1296 and https://github.com/lycheeverse/lychee/issues/1480
