v0.3.0

• The editor and playground are imported from @tsquare-js/react/editor, their own entry point, because they bundle CodeMirror (about 125 KB compressed): apps that import only @tsquare-js/react (boards, viewer, react-markdown) don't get it.
• <WireframeEditor>: tsquare's code editor (the playground's): highlighting, autocomplete, problems underlined about 300 ms after typing stops, Enter keeping the indentation, a whole pasted model reply trimmed to its code block, long lines wrapped (continuing 4 spaces past their indentation). value/defaultValue, onChange, onProblems, autocomplete, lineNumbers, lineWrapping, ariaLabel; a ref gets getText, setText, focus and goToLine. Built on tsquare/editor (tsquare 0.12), with CodeMirror bundled inside it.
• <WireframePlayground>: the editor and the viewer side by side on one text, half and half (stacked below 900px, or layout="row"/"column"). While someone types, the board redraws every 200 ms or so and problems wait; after a pause they're underlined in the editor and counted in a note on the board.
• The viewer stops when it unmounts (tsquare's new destroy()): no listeners, resize watching or board image left behind.
• Styles for the editor (its colors from the --tsquare-* theme; highlighting from --tsquare-syntax-*) and the playground; --tsquare-editor-height, --tsquare-playground-height.
• Docs: Editor and Playground pages; the overview's table and Styling cover them.
• Needs tsquare 0.12.1 or later.