Adds CI for Positron extension #1048

Job	Run time
jupyterlab / native (macos-latest)	4m 36s
jupyterlab / native (windows-latest)	8m 12s
agent / agent	2m 29s
build / build	3m 3s
jupyterlab / native (ubuntu-latest)	3m 32s
positron / docker	1m 45s
positron / native (macos-latest)	47s
web / web	1m 40s
positron / native (windows-latest)	1m 13s
positron / native (ubuntu-latest)	30s
bats / docker (ubuntu-latest)	1m 32s
bats / native (macos-latest)	22s
cypress / native (macos-latest)	1m 2s
bats / native (windows-latest)	1m 0s
cypress / native (windows-latest)	6m 52s
bats / native (ubuntu-latest)	22s
cypress / native (ubuntu-latest)	51s
	39m 48s

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