refactor: use schemapi from altair submodule

[manzt]

Prior to this PR, the code in tools/schemapi/* was copied from Altair. This PR adds Altair as a git submodule, and then imports the code from tools/altair/tools/schemapi.

I believe this is the "correct" way to keep things in sync with Altair (and allow us to pin versions if necessary), but perhaps it's overboard. Open to suggestions.

[sehilyi]

I like the idea of using a submodule, more than the previous approach.

I think we can also consider pinning a specific branch of released Altair versions, such as v4.2.X which is dedicated for v4.2.0 and any future patches (they create a new branch when bumping a version). For example, we can run

git config -f .gitmodules submodule.tools/altair.branch v4.2.X

git submodule update --remote 

Changes on .gitmodules:

[submodule "tools/altair"]
	path = tools/altair
	url = https://github.com/altair-viz/altair.git
+	branch = v4.2.X

This way, I think we ensure that we track a stable version of Altair, and it is more clear which Altair version we use by seeing the .gitmodules file.

[manzt]

I think it should be find to track commits on the main branch, but I see you point.The git submodule doesn't follow a branch in realtime but pins a specific commit. You must manually run git submodule update to update to the latest commit on the branch it tracks.

The other thing is that this isn't really following a version of Altair, but a version of an internal dependency (schemapi) so I'm not sure if following semver for Altair makes sense (since the internals could theoretically change).

[sehilyi]

Yes, it makes sense to track a specific commit. I guess I wondered about (1) when to pull the updates from the submodule to this repo (e.g., whenever there is a major update on schemapi or a new Altair version release?) and (2) if the schemapi in the main branch can be considered a stable code. Since (I believe) schemapi in a specific branch (e.g., v4.2.X) is used for the released Altair, I think we can consider it to be stable and is dedicated for v4.2.0. But, given that schemapi is not updated that frequently (i.e., I see there are no updates since v4.2.0 in December last year), this does not matter that much.

[manzt]

I think I'm going to merge this for now, and we can pin a specific branch if it becomes an issue!