Fix string.slice to handle zero gracefully

Author: JosephTLyons

src/gleam/string.gleam

@@ -188,7 +188,7 @@ fn less_than(a: String, b: String) -> Bool
 /// ```
 ///
 pub fn slice(from string: String, at_index idx: Int, length len: Int) -> String {
-  case len < 0 {
+  case len <= 0 {
     True -> ""
     False ->
       case idx < 0 {

test/gleam/string_test.gleam

@@ -335,6 +335,8 @@ pub fn slice_test() {
 
   assert string.slice("gleam", at_index: 2, length: -3) == ""
 
+  assert string.slice("gleam", at_index: 2, length: 0) == ""
+
   assert string.slice("👶🏿", at_index: 0, length: 3) == "👶🏿"
 }