fix: correctly remove indentation one line even if escaped line brake

[guibou]

Close #141.

[fmtTrim|
   foo\
   bar
|]

should be foobar instead of foo bar.

This is fixed by doing the whitespace trimming on the input string of
the quasiquotes (here \n foo\ bar) instead of the output string.

However it breaks two previous features:

• When an interpolation block had whitespace, it should now be indented at least as much as the string, otherwise the interpolation block indentation will be used as reference.
• Escaping the first line is now ignored. But actually, escaping the first line behaves exactly as not escaping the first line, so it breaks nothing.