Convert yaml.ts to handle multiple models per method

[koesie10]

This changes YAML parsing/creating functions for the model editor to handle multiple models per method. The changes in the actual YAML handling are fairly small because the format itself already supports multiple models per method.

I've introduced a few helper functions to convert between the old and new types. This should only be necessary while we're in the middle of the transition to the new types and can be removed later. For now, we'll just take the first model in the array when converting from the new to the old type. This is a change in the behavior since currently we always take the last model in the array but this behavior is undocumented and unsupported, so it should be fine to change it.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[robertbrignull]

LGTM

So the behaviour when creating YAML files is now that all modelings for a method are treated as a unit. So if a method already has modelings [A, B] but then we try to write the new modeling [C], it'll remove A and B and just write C, instead of writing [A, B, C]. I think that's correct and the tests do cover this.