feat(time): support promise test completion - #1015

Summary

• make mockTimeSource().run() return a Promise when called without a callback, while preserving the existing callback form
• add overloads for xstream, RxJS, and most entrypoints so await Time.run() type-checks
• document both callback and async/await usage for test frameworks such as AVA

Fixes #765

Verification

• node_modules\.bin\tsc.cmd --module commonjs --outDir .\lib\cjs
• node_modules\.bin\tsc.cmd --module es6 --outDir .\lib\es6
• node_modules\.bin\tslint.cmd --project tsconfig.lint.json --config ..\tslint.json
• node_modules\.bin\markdown-doctest.CMD (21 passed, 7 skipped)
• node_modules\.bin\mocha.cmd test/time.ts --require ts-node/register --grep promise-style --exit (6 passing)
• git diff --check

Note: the full node_modules\.bin\mocha.cmd test/**/*.ts --require ts-node/register --exit run reaches 118 passing tests including the new promise tests, then fails 12 existing unsubscribe timing assertions with the same [0, 1] vs [0] pattern across xstream/RxJS/most on this Windows + Node 22 environment. The focused promise tests pass independently.