Add UI tests for parse errors

[lambda-fairy]

For historical reasons, Maud doesn't have good test coverage for its diagnostics. In fact, at the moment there is only one such test 😔 The lack of tests will block further changes (e.g. #208, #215) so we should address this.

For a list of things to test, go to parse.rs and search for abort! and abort_call_site!.

[lambda-fairy]

• unexpected end of input - Add more UI tests #262
• @let only works inside a block
• unknown keyword @{}
• expected keyword after @
• found keyword {}
• invalid syntax
• expected string - Add more UI tests #262
• expected body for this @if
• expected body for this @else
• expected body for this @while
• missing in in @for loop
• expected body for this @for
• expected body for this @match
• unexpected end of @match pattern
• unexpected end of @match arm
• unexpected end of @let expression
• unexpected end of @let expression (are you missing a semicolon?)
• unexpected element
• element body must be wrapped in braces
• duplicate attribute {}