refactor(compiler): switch Binary.isAssignmentOperation to type guard function

Author: fisker

packages/compiler/src/expression_parser/ast.ts

@@ -268,6 +268,8 @@ export class Interpolation extends AST {
   }
 }
 
+type AssignmentOperation = '=' | '+=' | '-=' | '*=' | '/=' | '%=' | '**=' | '&&=' | '||=' | '??=';
+
 export class Binary extends AST {
   constructor(
     span: ParseSpan,
@@ -282,7 +284,7 @@ export class Binary extends AST {
     return visitor.visitBinary(this, context);
   }
 
-  static isAssignmentOperation(op: string): boolean {
+  static isAssignmentOperation(op: string): op is AssignmentOperation {
     return (
       op === '=' ||
       op === '+=' ||