Add new tiebreak: 'chunk'

Favors the line with shorter matched chunk. A chunk is a set of
consecutive non-whitespace characters.

Unlike the default `length`, this new scheme works well with tabular input.

  # length prefers item #1, because the whole line is shorter,
  # chunk prefers item #2, because the matched chunk ("foo") is shorter
  fzf --height=6 --header-lines=2 --tiebreak=chunk --reverse --query=fo << "EOF"
  N | Field1 | Field2 | Field3
  - | ------ | ------ | ------
  1 | hello  | foobar | baz
  2 | world  | foo    | bazbaz
  EOF

If the input does not contain any spaces, `chunk` is equivalent to
`length`. But we're not going to set it as the default because it is
computationally more expensive.

Close #2285
Close #2537
- Not the exact solution to --tiebreak=length not taking --nth into account,
  but this should work. And the added benefit is that it works well even
  when --nth is not provided.
- Adding a bonus point to the last character of a word didn't turn out great.
  The order of the result suddenly changes when you type in the last
  character in the word producing a jarring effect.

src/result.go

@@ -49,6 +49,21 @@ func buildResult(item *Item, offsets []Offset, score int) Result {
 		case byScore:
 			// Higher is better
 			val = math.MaxUint16 - util.AsUint16(score)
+		case byChunk:
+			b := minBegin
+			e := maxEnd
+			l := item.text.Length()
+			for ; b >= 1; b-- {
+				if unicode.IsSpace(item.text.Get(b - 1)) {
+					break
+				}
+			}
+			for ; e < l; e++ {
+				if unicode.IsSpace(item.text.Get(e)) {
+					break
+				}
+			}
+			val = util.AsUint16(e - b)
 		case byLength:
 			val = item.TrimLength()
 		case byBegin, byEnd: