Remove method and modeledMethods from SaveModeledMethods message

[robertbrignull]

Removes the methods and modeledMethods fields from SaveModeledMethods in favour of including methodSignatures. The extension host can use the ModelingStore to look up the methods and modelings using the method signatures.

Involves adding getMethods and getModeledMethods to ModelingStore.

Tested locally and seems to be saving models and updating the "unsaved" labels correctly.

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.

[charisk]

Looks good!