[xml] HtmlUtil#getStartTag: support tags with digits, like <h2>

GitOrigin-RevId: c3997423c38ade8a759aa086bc7d788c18e834b8

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/javadocBlankLines/beforeHtmlBlockTag3.java

@@ -0,0 +1,14 @@
+// "Insert <p>" "false"
+class Test {
+  /**
+   * Answer to the ultimate question of life, the universe, and everything
+   *<caret>
+   *  <h2>Subheader</h2>
+   * That's it. That's all there is.
+   *
+   * @return The number 42
+   */
+  int answer() {
+    return 42;
+  }
+}

xml/xml-psi-impl/src/com/intellij/xml/util/HtmlUtil.java

@@ -540,10 +540,11 @@ public final class HtmlUtil {
     if (startsWithTag(line)) {
       int tagStart = line.indexOf("<");
       if (tagStart >= 0) {
-        tagStart ++;
+        tagStart++;
         for (int i = tagStart; i < line.length(); i ++) {
-          if (!Character.isAlphabetic(line.charAt(i))) {
-            return line.substring(tagStart,i);
+          char ch = line.charAt(i);
+          if (!Character.isAlphabetic(ch) && !Character.isDigit(ch)) {
+            return line.substring(tagStart, i);
           }
         }
       }