Fix bugs and refactor

[lpotthast]

This PR addressess a few things:

• Adds a .gitattributes file, enforcing lf newline characters. This reduces churn when generating the library from different operating systems
• Constructing an SvgIcon is now done by calling SvgIcon::new
• Moved the git module to the build-crate root, as it has no relation to / dependency on icon packages
• Switched back from using inline_html to emitting the svg content directly. This keeps the <title> element as the first child.
• Moved the svg module inside the icon module.
• Updated the svg processing
  • Using fully configured EmitterConfig to have more control over the output
  • Attributes are now read semantically
  • Namespace information is not emitted for children
  • Enforce that newlines are not replaced by " " or " " (Windows): This allows us to not rely on inner_html!
  • Added test to ensure proper parsing of the svgs content
• Moved main processing to Library::generate, keeping main.rs cleaner
• Move writing of icon components from main processing to icon_module.rs
• Made PackageType#is_category use non-exhaustive match
• Using the typestate pattern for packages, only allowing icon reads after the package was downloaded
• Fixed clippy lints