fix!: Clarify Snippet API

[epage]

This is a batch API change from talking to @Muscraft about what would make the clearer, what will make it more easily adapt to rustc, and what will make it better encoded the design in the type system.

Renames:

• Snippet -> Message
• Slice -> Snippet
  • Slice::new to Snippet::source
• SourceAnnotation -> Annotation
• AnnotationType -> Level

Other breaking changes:

• Snippet::line_start is now defaulted to 1 instead of required
• Create Message from Level
• Create Annotation from Level
  • Annotation::label is now optional

New:

• Bulk-add functions to make it simpler to use builder API for automated conversion

What wasn't done:

• Deferred: Switch Annotation from using Level to a Style { Level, Neutral, Added, Removed }
• Deferred: Remove footer or convert it from Vec<Label> to Vec<Message>
• Deferred: Switching fields that are Option to have constructor methods that are Option
  • A little messy for people directly making a Message
  • Simplifies use with automated conversion
  • Alts
    • Leave as-is
    • Switch to &mut builder which gets messy with the nesting builder API
• Message::id to Message::code. I feel like the former is more semantically meaningful

[Muscraft]

Thanks for this! I think this is a good change overall.