Add \ion, \positiveion, \negativeion commands for chemistry notation

[drdrew42]

What

Adds three LaTeX commands for chemistry ion-charge notation:

• \ion[sign]{charge} — a superscript-only command: an editable charge block followed by a fixed +/- sign, rendering e.g. the "2+" of a 2+ cation.
• \positiveion / \negativeion — sign-bound shorthands that canonicalize to \ion[+]{…} / \ion[-]{…}.

Why

These commands existed in a downstream MathQuill fork used for WeBWorK chemistry problems — the chemistry answer-entry toolbars drive \positiveion/\negativeion for entering ion charges. They were lost when that work moved onto @openwebwork/mathquill, silently breaking those toolbar buttons. Restoring them upstream lets the downstream work drop the private fork.

How

Ion extends SupSub, mirroring the existing subscript/superscript classes in commands.ts — supsub = 'sup', an htmlTemplate carrying the editable charge block plus a .mq-ion-classed sign span, and a parser() built on latexMathParser.optBlock / .block exactly like NthRoot. latex() serializes to \ion[sign]{charge}; an absent optional sign defaults to +, and an empty charge block to 1.

.mq-ion is a styling hook on the sign span — no CSS rule is added; it renders correctly with the existing mq-sup styles.

Testing

• npm run build, npm run lint:check, and npm run format:check all pass.
• Round-trip parse tests added to test/latex.test.ts (the ion charges test), following the existing assertParsesLatex pattern.

🤖 Generated with Claude Code