Update mjcf parser and urdf parser

[etaoxing]

Category

• New feature
• Bugfix
• Breaking change
• Refactoring
• Documentation
• Other (please explain)

Description

(Upstreaming some changes made for Rewarped.)

Update parse_mjcf() and parse_urdf() so that they match better. MJCF parser is heavily modified to match URDF parser. There are minimal changes to the URDF parser, other than now supporting capsules and adding a hide_visuals kwarg.

Changelog

• Improve parse_mjcf() so that it better matches parse_urdf()
• Add support for <capsule> to parse_urdf()

Before your PR is "Ready for review"

• Do you agree to the terms under which contributions are accepted as described in Section 9 the Warp License?
• Necessary tests have been added
• Documentation is up-to-date
• Auto-generated files modified by compiling Warp and building the documentation have been updated (e.g. stubs.py, functions.rst)
• Code passes ruff check and ruff format --check?

[etaoxing]

@eric-heiden

I've tried to make the commit history readable, since this PR makes some large changes to parse_mjcf(), so hopefully the changes are easier to review.

I don't have the bandwidth to add tests, but I can share some MJCF or URDF files that were used if that helps.