Give spacing marks space (#17826)

Spacing marks are called so, because they have a positive advance
width, unlike their non-spacing neighbors (as the name indicates).
After this we stop assigning such gc=Mc codepoints a zero width.

Closes #17810

Author: lhecker

src/tools/GraphemeTableGen/Program.cs

@@ -161,7 +161,7 @@
 }
 buf.Append("};\n");
 
-buf.Append($"constexpr uint{trie.Stages[^1].Bits}_t ucdLookup(const char32_t cp) noexcept\n");
+buf.Append("constexpr int ucdLookup(const char32_t cp) noexcept\n");
 buf.Append("{\n");
 foreach (var stage in trie.Stages)
 {
@@ -290,11 +290,10 @@ static Ucd ExtractValuesFromUcd(string path)
             };
 
             // There's no "ea" attribute for "zero width" so we need to do that ourselves. This matches:
-            //   Mc: Mark, spacing combining
             //   Me: Mark, enclosing
             //   Mn: Mark, non-spacing
             //   Cf: Control, format
-            if (generalCategory.StartsWith("M") || generalCategory == "Cf")
+            if (generalCategory == "Me" || generalCategory == "Mn" || generalCategory == "Cf")
             {
                 width = CharacterWidth.ZeroWidth;
             }