pl-matrix-component-input simplification

[eliotwrobson]

See title. Part of #7027.

[eliotwrobson]

Going to go ahead and close this and extract some of the more useful components into a smaller PR. On further evaluation, the way that the pl-matrix-component-input element generates the HTML is kinda messy, and rather than extracting the functions doing this, it should be baked into a mustache template for a new element (perhaps just pl-matrix-input). Then, this element can have symbolic and numeric grading modes which grade relative to a reference matrix correct answer.

A lot of this is orthogonal to the work done here, and only really needs to be picked up if people find they really need symbolic matrix input.