Implement registerCompletionHandler() #1469

Job	Run time
Install dependencies without using a lockfile	29s
TypeScript compatibility (~5.2)	15s
TypeScript compatibility (~5.3)	15s
Lint source files	26s
Test package-lock for unexpected modifications	16s
Node.js (^18.18, ubuntu-latest)	3m 56s
Node.js (^18.18, windows-latest)	6m 49s
Node.js (^18.18, macos-latest)	6m 59s
Node.js (^20.8, ubuntu-latest)	6m 1s
Node.js (^20.8, windows-latest)	6m 42s
Node.js (^20.8, macos-latest)	6m 36s
Node.js (^21, ubuntu-latest)	6m 9s
Node.js (^21, windows-latest)	6m 33s
Node.js (^21, macos-latest)	5m 3s
	56m 29s

Provide feedback