🚩(frontend) add feature flag on "Copy as HTML"

As a blue print, we add a feature flag on
"Copy as HTML" button in the doc toolbox.
This feature flag is controlled by the `CopyAsHtml`
feature flag.

Be aware:
- if the feature flag is disabled, the button
will be shown
- if the feature flag is enabled and send true,
the button will be shown
- if the feature flag is enabled and send false,
the button will be hidden

src/frontend/apps/impress/jest.config.ts

@@ -23,6 +23,7 @@ const jestConfig = async () => {
       '\\.svg$': '<rootDir>/jest/mocks/svg.js',
       '^.+\\.svg\\?url$': `<rootDir>/jest/mocks/fileMock.js`,
       BlockNoteEditor: `<rootDir>/jest/mocks/ComponentMock.js`,
+      'custom-blocks': `<rootDir>/jest/mocks/ComponentMock.js`,
       ...nextJestConfig.moduleNameMapper,
     },
   };