Handle ! following a keyword in isClassMemberStart (#20617)

* Handle `!` following a keyword in isClassMemberStart

* Fix test

src/compiler/parser.ts

@@ -5462,6 +5462,7 @@ namespace ts {
                 switch (token()) {
                     case SyntaxKind.OpenParenToken:     // Method declaration
                     case SyntaxKind.LessThanToken:      // Generic Method declaration
+                    case SyntaxKind.ExclamationToken:   // Non-null assertion on property name
                     case SyntaxKind.ColonToken:         // Type Annotation for declaration
                     case SyntaxKind.EqualsToken:        // Initializer for declaration
                     case SyntaxKind.QuestionToken:      // Not valid, but permitted so that it gets caught later on.

tests/baselines/reference/parserIsClassMemberStart.js

@@ -0,0 +1,12 @@
+//// [parserIsClassMemberStart.ts]
+class C {
+    type!: number;
+}
+
+
+//// [parserIsClassMemberStart.js]
+var C = /** @class */ (function () {
+    function C() {
+    }
+    return C;
+}());

tests/baselines/reference/parserIsClassMemberStart.symbols

@@ -0,0 +1,8 @@
+=== tests/cases/compiler/parserIsClassMemberStart.ts ===
+class C {
+>C : Symbol(C, Decl(parserIsClassMemberStart.ts, 0, 0))
+
+    type!: number;
+>type : Symbol(C.type, Decl(parserIsClassMemberStart.ts, 0, 9))
+}
+

tests/baselines/reference/parserIsClassMemberStart.types

@@ -0,0 +1,8 @@
+=== tests/cases/compiler/parserIsClassMemberStart.ts ===
+class C {
+>C : C
+
+    type!: number;
+>type : number
+}
+

tests/cases/compiler/parserIsClassMemberStart.ts

@@ -0,0 +1,3 @@
+class C {
+    type!: number;
+}