Upgrade all dependencies

[benjie]

Description

• Node 18 now minimum supported version
• All dependencies updated
• TypeScript from v3 to v5; lots of changes required
• Upgraded TypeScript config to do Node18 stuff
• General refactoring
• Had to move some inline snapshots to external file due to Jest: Couldn't locate all inline snapshots.
• Use a version file rather than importing from package.json

Performance impact

Unknown.

Security impact

Hopefully an improvement?

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.