feat: parse <inertial> elements and joint effort/velocity limits

[ferrolho]

Supersedes #299 by @dkozma -- this PR is based on their work with review feedback addressed.

Changes

Inertial parsing (<link>)

• Parses <inertial> child elements: mass, origin (xyz + rpy), and inertia (ixx, ixy, ixz, iyy, iyz, izz)
• inertial is always present on URDFLink with URDF spec defaults (mass: 0, origin: identity, inertia: zeros), same pattern as limit on URDFJoint
• Uses number[] for origin xyz/rpy (consistent with processTuple)
• Added copy() to URDFLink for the new property
• Exported URDFInertial TypeScript interface

Joint limits (<joint>)

• Parses effort and velocity from <limit> elements
• Both default to 0 (not null)

Other

• Updated README documentation for .inertial on URDFLink and .limit on URDFJoint
• Updated tests (all 32 pass, 6 skipped, 1 todo -- all pre-existing)
• Dropped the meshPath addition from Add joint limit and inertial link data #299 (separate concern, LoadingManager is the right approach)

Credit

Original work by @dkozma in #299. Review feedback from @gkjohnson addressed in ferrolho@9d3c29a.

[gkjohnson]

This is great! Thanks for taking these changes to the finish line.