Schema change: `msItem/author` and `msItem/editor` no direct text nodes, must nest in `persName` elements

[wlpotter]

For author or editor elements that are direct children of an msItem element, which define the creators of the ms item, any names should be nested within a persName element rather than having any direct text node children.

Cf. https://bl.syriac.uk/documentation/guidelines#msitemAuthor for the editorial documentation related to this.

I'm working on the data changes for this, so we just want to constrain to ensure new edits get flagged if they don't follow this.