Creating stacks if they don't exist

[simenandre]

Pulumi Automation API supports creating stacks if they don't exist. I think it could be useful to make this possible in the Github Action too.

With it, users can build a workflow that does not require developers/contributors of a repository to have Pulumi installed for all operations. For example, if you want to spin up a stack (environment), for testing.

Fixes: #9

[simenandre]

I suggest that we add a new input (e.g. ensure), and when that is available, we use createOrSelectStack instead of selectStack here:

actions/src/main.ts

Lines 31 to 34 in c792c1e

	const stack = await LocalWorkspace.selectStack({
	stackName: config.stackName,
	workDir: workDir,
	});