Specify the specific BSD and AFL versions this repo is licensed under.

[Julian]

Historically JSON Schema has declared that contributions are licensed BSD or AFL, without specifying the version of BSD. We’ve been asked to specify a version for work elsewhere in the ecosystem, and OpenJS Foundation counsel recommended BSD 3-clause as the most common choice. This commit clarifies the specific versions (3-clause and v3.0 respectively), and adds a LICENSE file to make this explicit.

See https://github.com/json-schema-org/json-schema-spec/issues/1160#issuecomment-1123583441 for the full guidance.

Per the guidance, we'll leave this open for a few weeks (perhaps till August 31?) in case there are comments from any historical contributors, who I'll also now ping (with apologies to those of you who aren't interested). @fge @kriszyp @geraintluff @nickl- @garycourt.

SPDX short identifiers have not been added as part of this PR (and were optional in the comment there). Obviously we're free to add them later.

Closes: #1160