Proposal: allow raw string literals in attributes

[criloz]

Similar to raw string literal in rust https://rahul-thakoor.github.io/rust-raw-string-literals/

Reason

it will be easy to add attributes of the type key=value which has JSON or other kind of serialization formats as value

Examples

{data  =    r#"{
                       "name": "John Doe",
                       "age": 43,
                       "phones": [
                          "+44 1234567",
                           "+44 2345678"
                        ]
           }"#}
## hello world

{template  =    r##" <div class="advice">
            Raw strings are useful for some situations.
        </div>"##}
## hello world