Remove anyhow from the WIT lexer

[PhoebeSzmucer]

• Add 3 more variants to lex:Error - ControlCodepoint, DeprecatedCodepoint, ForbiddenCodepoint
• Delete anyhow from the lexer API and implementation, by using core::result::Result instead of anyhow::Result. This way the consumer of the lexer can extract granular error information without having to parse the error message.
• Introduce lex::Error::position()

Part of #2460

[alexcrichton]

Thanks!

[alexcrichton]

Could you add some docs to this method as well to indicate what the return value is? (e.g. a byte-offset from the start of the file)

[PhoebeSzmucer]

Done. These are byte offsets but they're global to the source map (which holds many files), not just to the file itself.