Feature request: provide ranges instead of just locations

[kubukoz]

The Smithy model provides SourceLocation in several places. E.g.:

• Shapes
• Nodes
• Traits.

However, a SourceLocation is just a single position in the file, and often it's not clear where a given piece of syntax ends.

It would be a nice addition to the ecosystem to have a (start, end) location pair, in something like a SourceRange. Tooling like LSPs and refactoring tools would have a nicer time if this was a thing.