Abort expression formats incorrectly

[DylanRJohnston]

Hey all,

I'm currently working on a transpiler from another language into VRL. All other parts of the AST implement Display in a way that is valid syntactically to parse it again i.e. parse <-> display (formatting not withstanding). However the implementation of Display for abort incorrectly places a colon in its output.

https://github.com/vectordotdev/vrl/blame/b6fd5ecf3e4d75ccc5a822dcebe8a0fe7d04ef78/src/parser/ast.rs#L1199

The following kind of test succeeds for all other expressions in VRL.

    #[test]
    fn abort_isomorphism_failure() {
        let expression = r#"abort "error message""#;

        assert_eq!(
            format!("{expression}\n"),
            vrl::parser::parse(expression).unwrap().to_string()
        )
    }

I don't know if this kind of isomorphism via std::fmt::Display is a guarantee the project is looking to provide, but beyond transpilers it's also very useful for pretty printers since printing from the parsed AST canonicalizes the formatting.