[mod] theme/simple: migrate codebase to TypeScript

TypeScript is a superset of JavaScript, converting the entire theme to
TypeScript allows us to receive much more feedback on possible issues made in
package updates or our own typos, furthermore, it allows to transpile properly
to lower specs. This PR couldn't be done in smaller commits, a lot of work
needed to make everything *work properly*:

- A browser baseline has been set that requires minimum **Chromium 93, Firefox
  92 and Safari 15** (proper visuals/operation on older browser versions is not
  guaranteed)
- LightningCSS now handles minification and prefix creation for CSS.
- All hardcoded polyfills and support for previous browser baseline versions
  have been removed.
- Convert codebase to TypeScript.
- Convert IIFE to ESM, handling globals with IIFE is cumbersome, ESM is the
  standard for virtually any use of JS nowadays.
- Vite now builds the theme without the need for `vite-plugin-static-copy`.
- `searxng.ready` now accepts an array of conditions for the callback to be
  executed.
- Replace `leaflet` with `ol` as there were some issues with proper Vite
  bundling.
- Merged `head` with `main` script, as head was too small now.
- Add `assertElement` to properly check the existence of critical DOM elements.
- `searxng.on` renamed to `searxng.listen` with some handling improvements.

docs/dev/makefile.rst

@@ -103,8 +103,8 @@ Node.js environment (``make node.env``)
 
    Node.js_ version {{version.node}} or higher is required to build the themes.
    If the requirement is not met, the build chain uses nvm_ (Node Version
-   Manager) to install latest LTS of Node.js_ locally: there is no need to
-   install nvm_ or npm_ on your system.
+   Manager) to install Node.js_ locally: there is no need to install
+   nvm_ or npm_ on your system.
 
 To install NVM_ and Node.js_ in once you can use :ref:`make nvm.nodejs`.
 
@@ -150,7 +150,7 @@ setup.
 ``make nvm.nodejs``
 -------------------
 
-Install latest Node.js_ LTS locally (uses nvm_)::
+Install latest Node.js_ locally (uses nvm_)::
 
   $ make nvm.nodejs
   INFO:  install (update) NVM at /share/searxng/.nvm