Parse \b and \f escape sequences correctly

[akiym]

\b and \f were left escaped, now they are correctly unescaped by from-json().

Before:

> from-json('"\\b\\f\\n\\r\\t"').raku
"\\b\\f\n\r\t"

After:

> from-json('"\\b\\f\\n\\r\\t"').raku
"\b\x[C]\n\r\t"

[antoniogamiz]

@timo could you merge this?

[timo]

merged and released to cpan, thank you very much, @akiym