readline: document emitKeypressEvents()

Pull Request check-list

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with this change (including linting)?
• Is the commit message formatted according to CONTRIBUTING.md?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

Affected core subsystem(s)

readline

Description of change

This PR is made up of two commits. The first adds documentation and a test for the already publicly available readline.emitKeypressEvents().

The second commit is semver major. Currently, 'keypress' events include the sequence and key info for named keys, but only the sequence for unnamed keys. This commit causes the key info to be included in both cases for increased consistency.

Refs: https://github.com/nodejs/node/issues/3836