Material for MkDocs 9

[squidfunk]

This PR contains all changes for the next major version of Material for MkDocs, which will be released as soon as the Carolina Reaper funding goal is hit. The biggest addition will be the new built-in search, which is finally released into the community edition. Sponsors have been using the new search for more than a year.

The following major changes are included (sponsors have been using most of them for over a year):

• Rewrite of search plugin with support for rich previews
• Detached search plugin implementation from MkDocs search plugin
• Added support for search tokenizer lookahead
• Added support for better search highlighting
• Added support for excluding content from search
• Added support for configurable search pipeline
• Added native support for offline search via offline plugin
• Omit code block line numbers from search
• Improved stability of search plugin when using --dirtyreload
• Removed support for prebuilding search indexes
• Removed support for indexing only titles in search
• Fixed detection of composition events in search interface
• Fixed search plugin not using title set via front matter
• Rewrite of worker communication layer

Additionally, since we're releasing a new major version, we take the opportunity to remove several deprecations and add some more feature flags to allow for more flexibility, the most important of which:

• Removed alternative admonition qualifiers in daffd08
• Removed :is selectors (in output) to allow for easier overriding of CSS in daffd08
• Removed legacy method for providing page title in feedback URL in 9979cbf
• Removed .title suffix on translations in 37250f1
• Added material namespace to plugins and updated to MkDocs 1.4.1 fee799a
• Added support for multiple instances of built-in tags plugin 17186b4
• Added support for removing copy-to-clipboard button in 128e267
• Added support for removing footer navigation in 8fdd1ad
• Updated to MkDocs 1.4.2 in 17186b4
• Updated Norwegian translations (via Fix: Norwegian translations #4647)
• Fixed Korean language code (kr ko) in 5aebaaa

[jrappen]

@squidfunk FYI, you still need a git tag

[squidfunk]

The branch was merged, I've not yet released it. Packing up.