AST improvement: remove unnecessary [node] boxing into arrays

[vadimkibana]

Summary

Closes elastic/kibana#222505

This PR specifically addresses the last part of the issue: [node] unboxing in AST elastic/kibana#222505 (comment) - removing unnecessary wrapping into arrays AST nodes.

BREAKING CHANGE: ES|QL AST and types shape has changed

Details

This PR adds breaking changes to the AST shape: once used in Kibana validation/autocomplete will need fixes. See description of breaking changes in the Changesets entry for this PR. In short:

• Some places where [node] was boxed in array are now plain node
• Some places represented nodes with syntax error used empty array [], now they are {type: 'unknown'} nodes.
• resolveItem(), singleItem() and other helpers used to work around these node-wrapping-into-array inconsistencies are now deprecated, and we should aim to remove them during next major bump.

Checklist

• Unit tests have been added or updated.
• The proper documentation has been added or updated.
• A changeset has been added (yarn changeset) if this change should be released. See docs/RELEASE.md.
• If this PR contains breaking changes, you have explained them using the BREAKING CHANGE: change syntax and selected a major bump in the changeset.
• The PR is opened as a draft until CI is green.