fix: allow keywords as Object member names

[claymcleod]

Clarified that reserved keywords remain valid as arbitrary keys in runtime, hints, and meta sections, metadata objects, and Object literals, and as Object member names. This made metadata such as import addressable through task.meta.import without treating the key as a declaration name.

This targeted WDL 1.2, where runtime access to task metadata was introduced. The change needs forward ports to wdl-1.3 and wdl-1.4; no backport to wdl-1.1 is needed. Closes #763.

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have considered whether the README.md or other documentation needs updating to account for these changes.
• You have updated the CHANGELOG.md describing the change and linking back to your pull request.
• You have read and agree to the CONTRIBUTING.md document.
• You have considered adding or updating relevant example WDL tests to the specification.
  • See the guide for more details.

For OpenWDL team members:

• Assign the appropriate individual to this PR.
• Triage the PR and add appropriate labels.