Added error for IdentifierStart immediately after a NumericLiteral

Fixes #4702.

src/compiler/diagnosticMessages.json

@@ -1011,6 +1011,10 @@
         "category": "Message",
         "code": 1350
     },
+    "An identifier cannot follow a numeric literal.": {
+        "category": "Error",
+        "code": 1351
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

src/compiler/scanner.ts

@@ -974,7 +974,10 @@ namespace ts {
             else {
                 result = text.substring(start, end); // No need to use all the fragments; no _ removal needed
             }
+
+            checkForIdentifierAfterNumericLiteral();
             if (decimalFragment !== undefined || tokenFlags & TokenFlags.Scientific) {
+                checkForIdentifierAfterNumericLiteral();
                 return {
                     type: SyntaxKind.NumericLiteral,
                     value: "" + +result // if value is not an integer, it can be safely coerced to a number
@@ -983,10 +986,17 @@ namespace ts {
             else {
                 tokenValue = result;
                 const type = checkBigIntSuffix(); // if value is an integer, check whether it is a bigint
+                checkForIdentifierAfterNumericLiteral();
                 return { type, value: tokenValue };
             }
         }
 
+        function checkForIdentifierAfterNumericLiteral() {
+            if (isIdentifierStart(text.charCodeAt(pos), languageVersion)) {
+                error(Diagnostics.An_identifier_cannot_follow_a_numeric_literal, pos, 1);
+            }
+        }
+
         function scanOctalDigits(): number {
             const start = pos;
             while (isOctalDigit(text.charCodeAt(pos))) {