New design, Typescript Rewrite, initial i18n support, Docusaurus alpha-73 support, improved yellow markings

[cmfcmf]

This PR includes

• the search experience now uses autocomplete.js v1-alpha-48, which features a modern design similar to that of the Algolia search autocomplete (fixes Match current Docusaurus search styling #32)
• support for translating/adjusting the placeholder text (fixes Allow to set search bar placeholder #29, fixes Customize search input placeholder #30)
• the yellow search term markings no longer use a URL parameter and disappear on navigation/reload (fixes Issues with Highlighting - How to stop the Highlighting? #28)
• requires/supports Docusaurus alpha-73
• improved keyboard support when selecting a search result (CTRL/SHIFT open the result in a new tab/window)
• a full rewrite to Typescript

See this GIF for a demo of how it looks like:

TODO

• looks really bad in dark mode
• the height of the autocomplete is a bit too much for the header
• update GIF in README
• autocomplete does not close if user is already at the URL of the search result Close detached modal after navigation algolia/autocomplete#547
• auto-detect language based on i18n settings
• "Cancel" button should be translated Support for internationalization algolia/autocomplete#548

[cmfcmf]

This PR is currently blocked by an upstream issue: algolia/autocomplete#547