Add a runner-agnostic BDD layer (two-go/bdd)

[tugkanboz]

Graduate the BDD helper from the examples into the package, but decoupled from any runner.

• given/when/then/and(text, fn) build step objects that share a world.
• scenario(steps) returns an async function you pass to your runner's test() (node:test, Jest, Vitest, Mocha). Does not import node:test.
• feature(name, scenarios) builds a labelled list you can loop and register.
• two-go/bdd subpath, types, unit tests, README, CHANGELOG.