fix: drop support for 'improvement' type (#899)

The 'improvement' type was never added to the spec, and isn't supported by other tools in the ecosystem.

BREAKING CHANGE: 'improvement' type will now be rejected by this config.

Author: gavinsharp

Diff for: @commitlint/config-conventional/index.js

@@ -24,7 +24,6 @@ module.exports = {
 				'docs',
 				'feat',
 				'fix',
-				'improvement',
 				'perf',
 				'refactor',
 				'revert',