With this change we reduce the reliance on the TypeScript target compiler option to output a certain ECMA version. Instead we now use the browsers that are configured in the Browserslist configuration to determine which ECMA features and version are needed. This is done by passing the transpiled TypeScript to Babel preset-env.
**Note about useDefineForClassFields**: while setting this to `false` will output JavaScript which is not spec compliant, this is needed because TypeScript introduced class fields many years before it was ratified in TC39. The latest version of the spec have a different runtime behavior to TypeScript’s implementation but the same syntax. Therefore, we opt-out from using upcoming ECMA runtime behavior to better support the ECO system and libraries that depend on the non spec compliant output. One of biggest case is usages of the deprecated `@Effect` decorator by NGRX and potentially other existing code as well which otherwise would cause runtime failures. Dropping `useDefineForClassFields` will be considered in a future major releases. For more information see: https://github.com/microsoft/TypeScript/issues/45995.
BREAKING CHANGE: Internally the Angular CLI now always set the TypeScript `target` to `ES2022` and `useDefineForClassFields` to `false` unless the target is set to `ES2022` or later in the TypeScript configuration. To control ECMA version and features use the Browerslist configuration.
With this change we update the TypeScript compilation target to `ES2020` for both new and existing projects. This is because all browsers that Angular supports (https://angular.io/guide/browser-support) support `ES2020` features without the need for polyfills.
rxjs 6 requires directory imports which are not support in ES modules. Disabling `fullySpecified` allows Webpack to ignore cases such as this until the package can be updated to use package exports.
