Support Janet's backtick strings

[MaxGyver83]

Support Janet's backtick strings. This fixes #47 (together with #49).

I think the difficult part is that strings may be wrapped in any number of backticks:

For long-strings where you don't want to type a lot of escape characters,
you can use 1 or more backticks (`) to delimit a string.
To close this string, simply repeat the opening sequence of backticks.

Janet Documentation: Strings, Keywords, and Symbols

This pull request handles strings wrapped in up to three backticks (on each side). For testing, delete the second line in this example code with dd:

(print
  (string/replace-all ``"`` `\"` ```"1"```))

Out of scope

Multiline strings are not handled in this pull request.

Deleting the first or the last line with dd fails in both of these forms, no matter if double quote or backtick strings are used:

(print (string ```
               A ``test`` case
               ``` ``!``))

(print (string "hello
               world
               !"))