npm(deps): bump mobx from 6.13.7 to 6.15.0

Bumps [mobx](https://github.com/mobxjs/mobx) from 6.13.7 to 6.15.0.
- [Release notes](https://github.com/mobxjs/mobx/releases)
- [Commits](https://github.com/mobxjs/mobx/commits)

---
updated-dependencies:
- dependency-name: mobx
  dependency-version: 6.15.0
  dependency-type: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]