Text spanner with centred text state of the art

[Andrew Bernard]

This exact question was asked on the list some years ago, but worth
asking again. What is the current best way/code for making text
spanners with text centred in the middle?

I have code developed by myself and Thomas Morley from a few years ago
but it seems to be behaving strangely now with random changes of font
size, something beyond my skill to debug. I hope there may be
something newer.

On this perennial topic, it would be great if this function could be
in baseline lilypond. It's in countless scores, and I know people want
it.

Andrew