For import completion of default import, convert module name to identifier (#19875)

* For import completion of default import, convert module name to identifier

* Suggestions from code review

src/compiler/scanner.ts

@@ -294,7 +294,7 @@ namespace ts {
     }
 
     /* @internal */
-    export function stringToToken(s: string): SyntaxKind {
+    export function stringToToken(s: string): SyntaxKind | undefined {
         return textToToken.get(s);
     }
 

src/compiler/types.ts

@@ -214,7 +214,7 @@ namespace ts {
         UndefinedKeyword,
         FromKeyword,
         GlobalKeyword,
-        OfKeyword, // LastKeyword and LastToken
+        OfKeyword, // LastKeyword and LastToken and LastContextualKeyword
 
         // Parse tree nodes
 
@@ -431,7 +431,9 @@ namespace ts {
         FirstJSDocNode = JSDocTypeExpression,
         LastJSDocNode = JSDocPropertyTag,
         FirstJSDocTagNode = JSDocTag,
-        LastJSDocTagNode = JSDocPropertyTag
+        LastJSDocTagNode = JSDocPropertyTag,
+        /* @internal */ FirstContextualKeyword = AbstractKeyword,
+        /* @internal */ LastContextualKeyword = OfKeyword,
     }
 
     export const enum NodeFlags {

src/compiler/utilities.ts

@@ -1905,6 +1905,14 @@ namespace ts {
         return SyntaxKind.FirstKeyword <= token && token <= SyntaxKind.LastKeyword;
     }
 
+    export function isContextualKeyword(token: SyntaxKind): boolean {
+        return SyntaxKind.FirstContextualKeyword <= token && token <= SyntaxKind.LastContextualKeyword;
+    }
+
+    export function isNonContextualKeyword(token: SyntaxKind): boolean {
+        return isKeyword(token) && !isContextualKeyword(token);
+    }
+
     export function isTrivia(token: SyntaxKind) {
         return SyntaxKind.FirstTriviaToken <= token && token <= SyntaxKind.LastTriviaToken;
     }