Better Completion for ExternalAutocomplete functions #1894

Job	Run time
Build on windows-latest for repo global.json	19m 51s
Build on windows-latest for 6.0 stable	20m 13s
Build on windows-latest for 7.0 preview	19m 59s
Build on macos-latest for repo global.json	16m 19s
Build on macos-latest for 6.0 stable	18m 43s
Build on macos-latest for 7.0 preview	18m 53s
Build on ubuntu-latest for repo global.json	14m 15s
Build on ubuntu-latest for 6.0 stable	15m 32s
Build on ubuntu-latest for 7.0 preview	13m 45s
	2h 37m 30s

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