Build: @storybook/source-loader migrated to strict TS

[usrrname]

lint(tsconfig.json): migrate @storybook/source-loader to use strict TS

Part of #22176

What I did

Change compilerOptions.strict from false => true in tsconfig.json of @storybook/source-loader

How to test

0. Ensure using node 16
1. Run yarn task --task compile --no-link --start-from=install
2. cd into src/code/libs/source-loader
3. Run yarn check
4. At root of project, run yarn test and ensure tests in source-loader pass.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[kasperpeulen]

LGTM!