Fix multiline blanks

[jmorag]

Before

[error]: Error with a multiline blank marker
     ╭──▶ test.zc@3:0-3:10
     │
   1 │ ╭┤ let id<a>(x : a) : a := x + 1
   2 │ │  <no line>
   3 │ ├┤ let const<a, b>(x : a, y : b) : a := x
     • │  ───────── 
     • │  ╰╸ After a multiline blank
     • │ 
     • 
─────╯

After

[error]: Error with a multiline blank marker
     ╭──▶ test.zc@3:0-3:10
     │
   1 │ ╭┤ let id<a>(x : a) : a := x + 1
   2 │ │  rec fix(f) := f(fix(f))
   3 │ ├┤ let const<a, b>(x : a, y : b) : a := x
     • │  ───────── 
     • │  ╰╸ After a multiline blank
     • │ 
     • 
─────╯

[jmorag]

I think this fixes #30 as well.