Perf: Performance optimization

[Alspb]

Description

Performance optimization:

1. Caches Node.length and Node.offset since these two methods are called very often (partly due to recursion inside) and cause significant lags for large documents. The trickiest thing here is to clear the cache when necessary.
2. Removes unnecessary Document.from and changes the first argument type in several methods from Delta document to Document document (which is anyway reasonable for a variable named "document").
3. Change a check including _root.toDelta() to assert.

Related Issues

• Fix [All] Various Performance Issues #997

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the package version in pubspec.yaml files.
• All existing and new tests are passing.
• I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[Alspb]

Checked the improvements for example x 50, i.e.
document: Document.fromJson([ for (var i = 0; i < 50; i++) quillTextSample ].expand((x) => x).toList()),
The editor works much faster, so that one can actually work with documents that large (about 70k characters).