FIX: Restore class-property babel transform for themes (#19883)

This seems to be required for decorators to work on class properties. Followup to 624f4a7de9

app/models/theme.rb

@@ -6,7 +6,7 @@ require "json_schemer"
 class Theme < ActiveRecord::Base
   include GlobalPath
 
-  BASE_COMPILER_VERSION = 70
+  BASE_COMPILER_VERSION = 71
 
   attr_accessor :child_components
 

lib/discourse_js_processor.rb

@@ -10,6 +10,8 @@ class DiscourseJsProcessor
   # babel: { debug: true } in ember-cli-build.js, then run `yarn ember build -prod`
   DISCOURSE_COMMON_BABEL_PLUGINS = [
     ["proposal-decorators", { legacy: true }],
+    "proposal-class-properties",
+    "proposal-private-methods",
     "proposal-class-static-block",
     "transform-parameters",
     "proposal-export-namespace-from",

spec/lib/discourse_js_processor_spec.rb

@@ -41,13 +41,6 @@ RSpec.describe DiscourseJsProcessor do
     script = <<~JS.chomp
       optional?.chaining;
       const template = func`test`;
-      class MyClass {
-        classProperty = 1;
-        #privateProperty = 1;
-        #privateMethod() {
-          console.log("hello world");
-        }
-      }
       let numericSeparator = 100_000_000;
       logicalAssignment ||= 2;
       nullishCoalescing ?? 'works';
@@ -76,6 +69,24 @@ RSpec.describe DiscourseJsProcessor do
     JS
   end
 
+  it "supports decorators and class properties without error" do
+    script = <<~JS.chomp
+      class MyClass {
+        classProperty = 1;
+        #privateProperty = 1;
+        #privateMethod() {
+          console.log("hello world");
+        }
+        @decorated
+        myMethod(){
+        }
+      }
+    JS
+
+    result = DiscourseJsProcessor.transpile(script, "blah", "blah/mymodule")
+    expect(result).to include("_applyDecoratedDescriptor")
+  end
+
   it "correctly transpiles widget hbs" do
     result = DiscourseJsProcessor.transpile(<<~JS, "blah", "blah/mymodule")
       import hbs from "discourse/widgets/hbs-compiler";