Adds CI for Positron extension #1051

Job	Run time
agent / agent	1m 56s
positron / docker	1m 29s
jupyterlab / native (macos-latest)	4m 9s
build / build	3m 40s
jupyterlab / native (windows-latest)	8m 10s
jupyterlab / native (ubuntu-latest)	2m 45s
positron / native (macos-latest)	59s
web / web	1m 17s
positron / native (windows-latest)	1m 43s
positron / native (ubuntu-latest)	29s
bats / docker (ubuntu-latest)	1m 25s
bats / native (macos-latest)	24s
cypress / native (macos-latest)	1m 29s
bats / native (windows-latest)	1m 7s
cypress / native (windows-latest)	4m 16s
bats / native (ubuntu-latest)	13s
cypress / native (ubuntu-latest)	48s
	36m 19s

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