fixing text mode behaviour and adding \math command

[Shrookin]

Summary

• add a \math{} command that parses its body as math inside prose, as the inverse of \text{}
• remember whether LaTeX entry began in text or free-text and restore that prose mode after bounded or unbounded completion
• allow nested LaTeX commands and spaces inside an open bounded entry, including \math{\forall x \land \frac{1}{2}}
• keep math placeholders in math mode until they are filled
• publish matching runtime bundles

Dependency

This PR is stacked on #3060 because free-text is introduced there. Until #3060 merges, GitHub will show its prerequisite commits in this PR; after it lands, this branch can be rebased and the diff will reduce to the text-mode behavior and \math{} changes.

Verification

• npx tsc --noEmit
• npx jest test --runInBand (335 passed)
• full Chromium physical-keyboard suite (48 passed)
• focused source/watch tests in Chromium, Firefox, and WebKit (9 passed)
• focused tests repeated against the checked-in dist/mathlive.mjs artifact in Chromium, Firefox, and WebKit (9 passed)
• targeted ESLint and Prettier checks

[arnog]

Can you expand on why the \math{} command is necessary, since \text{let $\forall x \in \R$} is already supported?

[Shrookin]

Of course. \math{} is not necessary for expressiveness; I proposed it primarily as an ergonomic counterpart to \text{}.
The motivation was interactive input in text-initialized fields. In free-text, neither typing nor pasting $...$ interprets it as math. In ordinary text mode, typing it also does not create a math region, although pasting a complete balanced expression does (which might be something unintended which, if confirmed, I am more than happy to address either with or separately from this PR). \text{let $\forall x \in \R$} works because the complete bounded \text{...} command originates in math mode and is parsed as a unit, not because $...$ acts as an interactive mode switch in a text-initialized field.
Since \text{} would not be the natural entry point in an already text-initialized field, I thought \math{} would provide intuitive symmetry: users familiar with either command could readily infer the other.
That said, I overlooked the existing \ensuremath{} command. It already covers the intended functional use case and has the advantage of being standard LaTeX. Therefore, \math{} would add discoverability and symmetry, but no new capability. I am happy to remove it and keep the PR focused on restoring prose mode after bounded and unbounded LaTeX input.