[items] ItemHistory: Add `previousStateTimestamp` method

[florian-h05]

Required for https://github.com/openhab/openhab-webui/issues/1597.

To Do

• [x] Update CHANGELOG
• [x] Update README

/cc @stefan-hoehn

[florian-h05]

@digitaldan I am now using the internal import (as before), I was able to fix the problem with typedefs & JSDoc. This is ready for merging now.