feat: fontApplyFeatures, and letterSpacing on createLayout spans and fontShapeText

[sidorares]

For sidorares/react-x11#588: tabular figures for a live readout, and spaced small caps. The ntk half is sidorares/ntk#357; react-x11 uses both once they are released.

fontApplyFeatures(font, features)

The font with OpenType features set by tag, through kCTFontFeatureSettingsAttribute with kCTFontOpenTypeFeatureTag — the sibling of fontApplyVariations:

• an array turns each tag on: ['tnum'];
• an object sets each tag to its value, true/false or a number for a feature that selects among alternates: { tnum: true, liga: false, salt: 2 }.

The settings replace any the font carries, the way a style names its features whole, and with nothing to set the font itself comes back — the same External — so a caller can apply it unconditionally. A non-font first argument is a TypeError, as for the other font verbs.

letterSpacing

Points added after every character, the last included — what kCTKernAttributeName does, and what CSS's letter-spacing long did; negative values tighten.

• a createLayout span option;
• an option to fontShapeText, as a third argument { letterSpacing }, so that a glyph run shaped alone and the paragraph it belongs to agree on every advance.

It says nothing about ligatures. CSS turns the optional ones off where letters are spaced, and a caller that wants that rule sets them through fontApplyFeatures on the spaced span — react-x11 will, to match ntk.

Tests

test/text-features.js, added to npm test: the system face's digits are proportional, one width under tnum in either form and still proportional under { tnum: false }; nothing to set answers the font itself; a non-font argument is a TypeError; a span's spacing adds three gaps to three characters, tightens when negative and spaces only its own span; fontShapeText spaces to the same width as createLayout, every advance carrying the gap, and without the option spaces nothing.

Mutation check, each change undone alone and rebuilt:

mutant	caught by
an array of tags adds nothing	tnum one width, the object form
false sets a feature on	{ tnum: false }
a span's letterSpacing ignored	four spacing checks
fontShapeText's option ignored	three fontShapeText checks
a copy made with nothing to set	the two identity checks

npm test passes on macOS 15 with the verbs built from this branch.