* [mod] client/simple: client plugins

Defines a new interface for client side *"plugins"* that coexist with server
side plugin system. Each plugin (e.g., `InfiniteScroll`) extends the base
`ts Plugin`. Client side plugins are independent and lazy‑loaded via `router.ts`
when their `load()` conditions are met. On each navigation request, all
applicable plugins are instanced.

Since these are client side plugins, we can only invoke them once DOM is fully
loaded. E.g. `Calculator` will not render a new `answer` block until fully
loaded and executed.

For some plugins, we might want to handle its availability in `settings.yml`
and toggle in UI, like we do for server side plugins. In that case, we extend
`py Plugin` instancing only the information and then checking client side if
[`settings.plugins`](https://github.com/inetol/searxng/blob/1ad832b1dc33f3f388da361ff2459b05dc86a164/client/simple/src/js/toolkit.ts#L134)
array has the plugin id.

* [mod] client/simple: rebuild static

SPDX short-form identifiers to communicate license information in a simple,
efficient, portable and machine-readable manner [1]

[1] https://spdx.dev/learn/handling-license-info/

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.