[Generator] Fixed wp-config salts replacement

Fixes:
* Regexp now properly handles carriage returns and/or newlines (since javascript doesn't match them with dots)
* Separate literal string replace (as `split(match).join(replace)`) since generated salts could contain "$" strings interpreted as backrefs

That second point works around a baffling javascript bug where `String.replace()` with a literal string match still interpolates special backrefs:

```js
"foo bar baz".replace('bar', '$`')
# "foo foo  baz"
```