docs: document accepted range whitespace

[vibhor-aggr]

Summary

• document whitespace accepted before partial versions in the range BNF
• document the accepted ~> tilde form
• allow one or more spaces between simple range expressions in the grammar

The README and standalone range.bnf now describe the same forms already accepted by the strict parser. This does not change runtime behavior.

Fixes #392.

Verification

• strict-mode smoke checks for >= 1.2.3 < 1.6.0, ^ 1.2.3 || ^ 2.4.6, ~ > 1.2.3, and ~> 1.2.3
• ./node_modules/.bin/tap test/ranges/valid.js
• npm test
• git diff --check