Bump proj4 and @types/proj4 (#352)

Bumps [proj4](https://github.com/proj4js/proj4js) and [@types/proj4](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/proj4). These dependencies needed to be updated together.

Updates `proj4` from 2.12.1 to 2.15.0
- [Release notes](https://github.com/proj4js/proj4js/releases)
- [Changelog](https://github.com/proj4js/proj4js/blob/master/changelog.md)
- [Commits](proj4js/proj4js@v2.12.1...v2.15.0)

Updates `@types/proj4` from 2.5.5 to 2.5.6
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/proj4)

---
updated-dependencies:
- dependency-name: proj4
  dependency-type: direct:development
  update-type: version-update:semver-minor
- dependency-name: "@types/proj4"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>