exposing pie insert text function

[archmoj]

This PR refactors and exposes pie insert-text function which appear to be useful in adding textinfo feature for waterfall & funnel in the next release.
See #3777, #3504 and #3504 (comment).

@plotly/plotly_js

[etpinard]

Hmm. I'm not this is worth pursuing. Here's a couple reasons:

• the flags for bar and waterfall will be different, so as is we can't just ♻️ insideText
• I'm not a fan of computing the full on-graph text content in calc, I would prefer having this in plot like for sunburst traces.

[archmoj]

Great! Closing in the favour of having this in plot.