Fix #4500 Allow keyword start after numbers

Author: benkobalog

compiler/src/dotty/tools/dotc/printing/SyntaxHighlighting.scala

@@ -79,7 +79,7 @@ object SyntaxHighlighting {
         } else {
           if (n.isUpper && (keywordStart || prev == '.')) {
             appendWhile(n, !typeEnders.contains(_), typeDef)
-          } else if (keywordStart) {
+          } else if (keywordStart || prev.isDigit) {
             append(n, keywords.contains(_), { kw =>
               if (kw == "new") typeDef(kw) else keyword(kw)
             })

compiler/test/dotty/tools/dotc/printing/SyntaxHighlightingTests.scala

@@ -60,6 +60,7 @@ class SyntaxHighlightingTests {
   @Test
   def expressions = {
     test("val x = 1 + 2 + 3", "<K|val> <V|x> = <L|1> + <L|2> + <L|3>")
+    test("if(true) 3 else 1", "<K|if>(<K|true>) <L|3> <K|else> <L|1>")
   }
 
   @Test