Implement remaining Plot widget changes

While most of the suggested changes to the [Plot widget PR](https://github.com/dls-controls/cs-web-lib/pull/15) have been made, there are still some outstanding changes not yet implemented. These are:

- [ ] Possibly extract part of `opiParser.ts` function `parseMultipleNamedProps` to make it reusable for other functions
- [ ] Be more precise for types of `obj` and `props` in `parseMultipleNamedProps`, rather than using `any`
- [ ] Consider whether we want to apply the same limit on maximum number of traces (20) as CSStudio does
- [ ] Currently only support plotting waveform PVs - we want to be able to plot time series data - this involves getting the data concatenation working as well as modify the way incoming data is handled
- [ ] Decide whether to use test() or it() for unit tests and enforce this change (this is not necessarily related to the plot widget specifically, but worth mentioning)
- [ ] Add tests for xyPlot.ts

Hopefully by implementing these changes, the Plot widget would be complete.