Raw strings are not supported in `#[footer]` attrs

[parasyte]

This is a limitation in LiteralExt::as_string(): https://github.com/parasyte/myn/blob/680794d1ea0052fc6fd1a7b9b20e77ae544bb8da/src/traits.rs#L193-L207

Should be fairly easy to fix. The parser just needs to be taught about raw string syntax, including arbitrary # characters surrounding the string literal.

Some examples:

#[footer = r"Boring raw string literal."]
#[footer = r#"A raw string literal that ignores "double quotes"."#]
#[footer = r##"A raw string literal that ignores #"hashed double quotes"#."##]
#[footer = r###"A raw string literal that ignores ##"double hashed double quotes"##."###]
// etc.

The current workaround is to use escape sequences in normal string literals.