fix(model): make .changed() deep aware

[SimonSchick]

Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Did you follow the commit message conventions explained in CONTRIBUTING.md?

Description of change

Fixes #10848

@sushantdhiman the problem here is that this change would be in conflict with current behaviour, we could deep clone complex values values when storing the previous values, this probably has some performance implications tho.

Imo we should suggest to consumers that when they edit nested properties that they have to call changed('key', true) manually.

[codecov]

Codecov Report

Merging #10851 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10851      +/-   ##
==========================================
+ Coverage   96.27%   96.27%   +<.01%     
==========================================
  Files          94       94              
  Lines        9186     9190       +4     
==========================================
+ Hits         8844     8848       +4     
  Misses        342      342

Impacted Files	Coverage Δ
lib/model.js	96.47% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 481bddd...4396bda. Read the comment docs.