feat: Add 'sort by function'

[claremacrae]

Description

Enable sort by function to return a sort key, which is then used for sorting tasks in search results.

Note:

• The tests (and hence also behaviour) and docs are not yet up to (my) release standards.
• I'm merging it now so that I can use it in my main vault, to confirm I'm happy with the behaviour, and to try to come up with some more realistic examples for the docs.

Motivation and Context

Partial implementation of #1479.

How has this been tested?

• Adding more tests
• Exploratory testing

Screenshots (if appropriate)

Types of changes

Changes visible to users:

• New feature (prefix: feat - non-breaking change which adds functionality)
• Documentation (prefix: docs - improvements to any documentation content for users)

Internal changes:

• Tests (prefix: test - additions and improvements to unit tests and the smoke tests)

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change has adequate Unit Test coverage.

Terms

• My contribution follow this project's contributing guide
• I agree to follow this project's Code of Conduct