fix(types): add isSoftDeleted to Model

[todd]

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 update the typescript typings accordingly (if applicable)?
• Did you follow the commit message conventions explained in CONTRIBUTING.md?

Description of change

The TypeScript declarations currently lack a signature for isSoftDeleted on Model. This PR adds that signature.

[papb]

Hello! I see you are a first-time contributor, thank you for taking the time to help Sequelize! I hope to see more PRs from you in the future!

Please update your branch though, it is out-of-date with the master branch

[codecov]

Codecov Report

Merging #11628 into master will decrease coverage by 6.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11628      +/-   ##
==========================================
- Coverage   96.26%   89.88%   -6.39%     
==========================================
  Files          94       92       -2     
  Lines        9190     9094      -96     
==========================================
- Hits         8847     8174     -673     
- Misses        343      920     +577

Impacted Files	Coverage Δ
lib/dialects/mssql/query-generator.js	2.62% <0%> (-93.3%)	⬇️
lib/dialects/mssql/query-interface.js	8.69% <0%> (-91.31%)	⬇️
lib/dialects/mssql/query.js	5.58% <0%> (-89.95%)	⬇️
lib/dialects/mssql/resource-lock.js	22.22% <0%> (-77.78%)	⬇️
lib/dialects/mssql/data-types.js	30.12% <0%> (-69.88%)	⬇️
...dialects/abstract/query-generator/helpers/quote.js	93.33% <0%> (-6.67%)	⬇️
lib/dialects/abstract/query-generator.js	95.81% <0%> (-1.2%)	⬇️
lib/sequelize.js	95.31% <0%> (-0.63%)	⬇️
lib/query-interface.js	91.7% <0%> (-0.49%)	⬇️
lib/model.js	96.47% <0%> (-0.07%)	⬇️
... and 2 more

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