fix(truncatewords): ignore leading whitespace when counting words (#954)

str.split(/\s+/) yields a leading empty element when the input starts with
whitespace, so that element was counted as a word and joined back into the
output: "  Ground control to Major Tom." truncated to 3 words returned
" Ground control..." instead of "Ground control to...".

Ruby Liquid splits in awk mode, which discards leading whitespace but keeps
trailing whitespace, so trimStart matches it and trim would not.

test/integration/filters/string.spec.ts

@@ -233,6 +233,12 @@ describe('filters/string', function () {
     it('should default len to 15', function () {
       return test('{{ "1 2 3 4 5 6 7 8 9 a b c d e f" | truncatewords }}', '1 2 3 4 5 6 7 8 9 a b c d e f...')
     })
+    it('should ignore leading whitespace when counting words', function () {
+      return test('{{ "  Ground control to Major Tom." | truncatewords: 3 }}', 'Ground control to...')
+    })
+    it('should keep trailing whitespace when not truncating', function () {
+      return test('{{ "1 2 3 " | truncatewords: 5 }}', '1 2 3 ')
+    })
   })
   describe('remove_last', function () {
     it('should remove the last occurrence of substring', function () {