Improve package tags editor

Today, the set of tags added to a package is given as a single string in a text box.

We explain this is intended to be semicolon delimited, but clearly we didn't get that right in the past as our own MS.VS.PS.Managed.VS package has:


We could add a new kind of editor for semicolon-delimited strings like this, similar to what we do for name/value pairs.

Start by locating the existing name/value pair editor used in the package editor and inspect how package tags are currently edited. The work is complete when semicolon-delimited tags can be edited as separate values without producing malformed tag strings, with the relevant editor behavior verified.

Written by the indexing model from the issue text.