Template literal types and mapped type 'as' clauses (#40336)

* Initial implementation of string template types

* Accept new API baselines

* Accept new baselines

* Unified checking for large cross product union types

* Accept new baselines

* Ensure errors from union type resolution are reported

* Accept new baselines

* Compute constraints for string template types

* Support `as T` clause in mapped types

* Accept new API baselines

* Add missing semicolon

* Add checking of `as T` clauses

* Support casing modifiers in string template types

* Accept new baselines

* Bump keyword maximum length

* fix anders

* Revert "fix anders"

This reverts commit b3178d4618.

* Properly handle 'as T' clause with keyof for mapped type

* Fix lint error

* Single character inferences and anchored end span matching

* Fewer array copy operations in template literal type resolution

* Handle cases where 'as T' maps multiple properties onto one

* Fix lint error

* Store key type instead of type mapper in MappedSymbol

* No constraint on `in T` type when `as N` clause present

* Rename from TemplateType to TemplateLiteralType

* Accept new API baselines

* Add tests

* Accept new baselines

* Address CR feedback

* Accept new API baselines

Co-authored-by: Erich Gamma <egamma@microsoft.com>

src/services/codefixes/convertLiteralTypeToMappedType.ts

@@ -48,6 +48,6 @@ namespace ts.codefix {
 
     function doChange(changes: textChanges.ChangeTracker, sourceFile: SourceFile, { container, typeNode, constraint, name }: Info): void {
         changes.replaceNode(sourceFile, container, factory.createMappedTypeNode(/*readonlyToken*/ undefined,
-            factory.createTypeParameterDeclaration(name, factory.createTypeReferenceNode(constraint)), /*questionToken*/ undefined, typeNode));
+            factory.createTypeParameterDeclaration(name, factory.createTypeReferenceNode(constraint)), /*nameType*/ undefined, /*questionToken*/ undefined, typeNode));
     }
 }

src/services/codefixes/convertToMappedObjectType.ts

@@ -44,6 +44,7 @@ namespace ts.codefix {
         const mappedIntersectionType = factory.createMappedTypeNode(
             hasEffectiveReadonlyModifier(indexSignature) ? factory.createModifier(SyntaxKind.ReadonlyKeyword) : undefined,
             mappedTypeParameter,
+            /*nameType*/ undefined,
             indexSignature.questionToken,
             indexSignature.type);
         const intersectionType = factory.createIntersectionTypeNode([