refactor(compiler): refactor template binding parsing (#23460)

A long time ago Angular used to support both those attribute notations:
- `*attr='binding'`
- `template=`attr: binding`

Because the last notation has been dropped we can refactor the binding parsing.
Source maps will benefit from that as no `attr:` prefix is added artificialy any
more.

PR Close #23460

packages/compiler/src/expression_parser/ast.ts

@@ -215,7 +215,7 @@ export class ASTWithSource extends AST {
 export class TemplateBinding {
   constructor(
       public span: ParseSpan, public key: string, public keyIsVar: boolean, public name: string,
-      public expression: ASTWithSource) {}
+      public expression: ASTWithSource|null) {}
 }
 
 export interface AstVisitor {