Allow suffix match on --nth with custom --delimiter

When --nth is used with a custom --delimiter, the last delimiter was
included in the search scope, forcing you to write the delimiter in
a suffix-match query. This commit removes the last delimiter from the
search scope.

  # No need to write 'bar,$'
  echo foo,bar,baz | fzf --delimiter , --nth 2 --filter 'bar$'

This can be seen as a breaking change, but I'm gonna say it's a bug fix.

Fix #3983

src/tokenizer.go

@@ -78,6 +78,11 @@ type Delimiter struct {
 	str   *string
 }
 
+// IsAwk returns true if the delimiter is an AWK-style delimiter
+func (d Delimiter) IsAwk() bool {
+	return d.regex == nil && d.str == nil
+}
+
 // String returns the string representation of a Delimiter.
 func (d Delimiter) String() string {
 	return fmt.Sprintf("Delimiter{regex: %v, str: &%q}", d.regex, *d.str)