Initial double-quote in a triple-quoted literal should be escaped

In the code `""""text"""`, it's hard to tell there's a typo. Formatting this as `"""\"text"""` even though it's not strictly necessary would help readability.

(Note: double-quotes at the end of triple-quoted strings are escaped because otherwise it would be parsed by elm-compiler as starting the closing delimiter.)

Start by locating the formatter entry point that handles Elm triple-quoted string literals, using the examples `""""text"""` and `"""\"text"""` as the expected case. Confirm the formatted output escapes the initial double quote for readability while preserving the existing handling of quotes at the closing delimiter.

Written by the indexing model from the issue text.