ApexUuid Versions Save

• Converted the repo to be an sfdx project, bumped metadata to API v56.0
• Created a new unlocked package (no namespace) v2.1.0
  • The previous release number for the repo is 2.0.0 - I haven't changed any functionality in this release, but I'm bumping the repo version number to v2.1.0 since releasing an unlocked package is a somewhat notable feature.
  • I also like to keep unlocked package version numbers in sync with the repo version number, so the unlocked package is starting at v2.1.0, even though it's the first release

Orgs that have previously deployed the Uuid and Uuid_Tests can install this package on top of your existing metadata - the existing versions of the classes will be "absorbed" into the unlocked package, and you can then easily upgrade when new versions are released.

1. Completely rewrote UUID generation to comply with UUID v4 standard
2. Renamed project & class from Guid to Uuid (since UUID is the true standard)
3. Added package.xml and Travis CI builds
4. Rewrote README.md to include use cases & code samples