Export usePlotBandLine to be used as extension point

[anajavi]

This allows library users to extend plotline within their own components.

As discussed in #228 (comment)

[codecov-io]

Codecov Report

Merging #237 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #237   +/-   ##
======================================
  Coverage    90.9%   90.9%           
======================================
  Files          68      69    +1     
  Lines         869     869           
  Branches      158     158           
======================================
  Hits          790     790           
  Misses         66      66           
  Partials       13      13

Impacted Files	Coverage Δ
...highcharts/src/components/PlotBandLine/PlotBand.js	100% <ø> (ø)	⬆️
packages/react-jsx-highcharts/src/index.js	100% <ø> (ø)	⬆️
...highcharts/src/components/PlotBandLine/PlotLine.js	100% <ø> (ø)	⬆️
...omponents/PlotBandLine/UsePlotBandLineLifecycle.js	100% <100%> (ø)
...sx-highcharts/src/components/PlotBandLine/index.js	100% <100%> (ø)	⬆️
...rts/src/components/PlotBandLine/UsePlotBandLine.js	100% <100%> (ø)
...s/src/components/PlotBandLine/PlotBandLineLabel.js	100% <100%> (ø)	⬆️

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[whawker]

Should the usePlotBandLineLifecycle.js file begin with a capital 'U' for consistency?

[anajavi]

Should the usePlotBandLineLifecycle.js file begin with a capital 'U' for consistency?

I can rename all these. I just named it after a function name, as react kind of forces hook functions to start with uncapitalized 'use'.

During the rewrite I also thought about flattening the src directory structure, because most of the components don't need index.js anymore. Decided to suggest that later, as now it is still easy to see changes from classes to hooks on git.