Adds CI for Positron extension #1047

Job	Run time
agent / agent	1m 57s
positron / docker	1m 20s
build / build	2m 53s
positron / native (macos-latest)	1m 12s
web / web	1m 33s
positron / native (windows-latest)	1m 42s
jupyterlab / native (macos-latest)	4m 11s
jupyterlab / native (windows-latest)	4m 8s
jupyterlab / native (ubuntu-latest)	3m 31s
positron / native (ubuntu-latest)	29s
bats / docker (ubuntu-latest)	1m 25s
bats / native (macos-latest)	23s
cypress / native (macos-latest)	53s
cypress / native (windows-latest)	1m 6s
bats / native (windows-latest)	54s
cypress / native (ubuntu-latest)	55s
bats / native (ubuntu-latest)	12s
	28m 44s

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