feat(framework): add default values for language/theme

[nnaydenow]

Currently, users are not able to set language and theme to their default values because there is no mechanics for that.

With this PR we are introducing new getDefaultLanguage / getDefaultTheme method which returns the default value for language / theme.

To reset your language to the default value:

import { setLanguage, getDefaultLanguage } from "@ui5/webcomponents-base/dist/config/Language.js";
setLanguage(getDefaultLanguage());

To reset your theme to the default value:

import { setTheme, getDefaultTheme } from "@ui5/webcomponents-base/dist/config/Theme.js";
setTheme(getDefaultTheme());

Fixes: #6712