vim-patch:43725ac: runtime(css): Add additional CSS pseudo-classes and functions (#42192)

Problem:  Other than `state()` these are widely supported CSS features
Solution: Add :host, :host-context(), slotted(), and state() CSS
          pseudo-classes and functions (S0AndS0).

Reference:
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:host
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:host_function
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/::slotted
- https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:state

TLDR: `host` and `host-context()` are useful for shadowDom stuff,
`::slotted()` enables targeting elements passed/injected into a
`<template>` and `state()` enables targeting custom identifiers.

For example, currently I'm (ab)using `:host` as a drop-in replacement
for `:root` when defining variables that are intended to be shared
across custom/template elements;

- `./assets/css/host.css`
   ```css
   :host {
     --default__margin-block: 0.25rem;
   }
   ```
- `./custom-elements/example.html`
   ```html
   <template>
     <link rel="stylesheet" href="/assets/css/host.css">

     <style>
       header {
         margin-block-start: calc(var(--default__margin-block) * 2);
       }
     </style>

     <header></header>
   </template>
   ```

closes: vim/vim#21404

https://github.com/vim/vim/commit/43725acce680d1b999e22fad68cea99c2b593325

Co-authored-by: S0AndS0 <strangerthanbland@gmail.com>

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