Auto merge of #13914 - TooManyBees:13894-use-graphemes, r=emilio

13894 Use graphenes in text-emphasis-style

<!-- Please describe your changes on the following line: -->
Use `unicode_segmentation` crate to truncate strings on grapheme boundaries.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13894 (github issue number if applicable).

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- [x] There are tests for these changes

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components/style/properties/longhand/inherited_text.mako.rs

@@ -741,6 +741,7 @@ ${helpers.single_keyword("text-align-last",
     use computed_values::writing_mode::T as writing_mode;
     use cssparser::ToCss;
     use std::fmt;
+    use unicode_segmentation::UnicodeSegmentation;
     use values::LocalToCss;
     use values::NoViewportPercentage;
 
@@ -889,7 +890,9 @@ ${helpers.single_keyword("text-align-last",
                 },
                 SpecifiedValue::None => computed_value::T::None,
                 SpecifiedValue::String(ref s) => {
-                    let string = s.chars().next().as_ref().map(ToString::to_string).unwrap_or_default();
+                    // Passing `true` to iterate over extended grapheme clusters, following
+                    // recommendation at http://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
+                    let string = s.graphemes(true).next().unwrap_or("").to_string();
                     computed_value::T::String(string)
                 }
             }