feat: test case coverage for digest, salt

[gjj]

For the digest process:

• Added illegalCharactersCheck, to be done before salting the document
  • Checks for ., [ and ] to prevent such characters from being salted since they would cause shadowed keys
  • I didn't put the checks inside salt due to how ${path}.${key} is being recursively passed into deepMap
• Fixed the supposed non-deterministic salts, changed it to using hardcoded salts
• Added number and boolean as allowed data types, and null is accepted now
  • Allowed data types are: object, array, string, number, boolean and values: null, everything else is rejected
• Improved test cases expected values

For schema:

• Since null is accepted, changed some test cases expected values (should now be expecting schema validation errors)

Also added test case coverage for:

• shadowed keys (arrays and objects)
• null and undefined values in document
• sparse arrays