Add project attribute to workspaces block

[dbanck]

Closes #206

Example UX

[radeksimko]

LGTM except for the descriptions as mentioned in-line.

Side note:
We will also need to regenerate the versions here to "expose" it

terraform-schema/schema/versions_gen.go

Lines 9 to 13 in bdb076b

	OldestAvailableVersion = version.Must(version.NewVersion("0.12.0"))
	LatestAvailableVersion = version.Must(version.NewVersion("1.5.0"))
	terraformVersions = version.Collection{
	version.Must(version.NewVersion("1.5.0")),

And on a related note, we probably shouldn't be setting beta versions as latest when generating the file, meaning that we should be able to provide schema relevant to pre-releases such as 1.6.0-beta3 but only if the configuration explicitly opts into it or the user has installed that version, otherwise we should assume latest stable version. I'm happy to file this as a separate issue or raise a PR unless you plan to do it (obviously not part of this PR either way).

[dbanck]

Side note: We will also need to regenerate the versions here to "expose" it

I added a TODO for later this week to regenerate the versions :)

[radeksimko]