feat(core): change Formly internal extensions priority to <=-1 (#3226)

demo/src/app/guides/custom-formly-extension.md

@@ -103,7 +103,7 @@ When registering an extension with the `FormlyModule`, you can provide an additi
 This will be used to change the order in which extensions are executed. If you have multiple extensions that change the same properties of a `FormlyFieldConfig`, you can use this to ensure they are executed in the correct order. Extensions with higher `priority` values will be executed later.
 
 If you don't provide a `priority` option, a default value of `1` will be used. <br/>
-Formly's own internal extensions have a priority of `-1`.
+Formly's own internal extensions have a priority of `<= -1`.
 
 ```typescript
   FormlyModule.forRoot({

src/core/src/lib/core.module.ts

@@ -23,10 +23,10 @@ export function defaultFormlyConfig(config: FormlyConfig): ConfigOption {
       { name: 'formly-template', component: FormlyTemplateType },
     ],
     extensions: [
-      { name: 'core', extension: new CoreExtension(config), priority: -1 },
-      { name: 'field-validation', extension: new FieldValidationExtension(config), priority: -1 },
-      { name: 'field-form', extension: new FieldFormExtension(), priority: -1 },
-      { name: 'field-expression', extension: new FieldExpressionExtension(), priority: -1 },
+      { name: 'core', extension: new CoreExtension(config), priority: -250 },
+      { name: 'field-validation', extension: new FieldValidationExtension(config), priority: -200 },
+      { name: 'field-form', extension: new FieldFormExtension(), priority: -150 },
+      { name: 'field-expression', extension: new FieldExpressionExtension(), priority: -100 },
     ],
   };
 }

src/core/src/lib/services/formly.config.ts

@@ -224,7 +224,7 @@ export class FormlyConfig {
     // flatten extensions object with sorted keys
     this.extensions = Object.keys(this.extensionsByPriority)
       .map(Number)
-      .sort()
+      .sort((a, b) => a - b)
       .reduce(
         (acc, prio) => ({
           ...acc,