Update configuration schema, extension metadata, and README

[addisonElliott]

Changes

• Add keywords and description for extension
  • Should aid with discovery and usage of this extension
• Specify defaults in extension contributes configuration
  • Settings will now autocomplete with the default value (see GIF below)
  • Remove defaults from code and use defaults populated from JSON schema
• Disallow additional properties in configuration
  • Unknown properties will now be flagged as invalid.
• Set command and name properties as required for each command button
• Add descriptions for each configuration option
  • This documentation should aid users in setting up the extension
• Add missing properties for command items
  • Options useVsCodeApi and focus were missing from configuration schema
• Remove CHANGELOG.md in favor of release notes in README.md.
  • Changelog was out-of-date and hasn't been touched in a few years.
• Refactor & update README
  • Minor whitespace and formatting fixes to the README
  • Added all configuration options to README with descriptions matching the JSON schema for consistency

[addisonElliott]

@seunlanlege Thanks for the awesome extension! Hopefully this PR isn't too large. I have a few others I'm going to submit after this but they'll depend on this one being merged.

[addisonElliott]

@seunlanlege can you review these PRs please?

[seunlanlege]

Thanks for the contribution