Support more detailed ligature types in terminal

[Tyriar]

From @joshlawton in #34103 (comment)

Just to confirm, this setting only accepts a boolean value and does not enable explicit font-feature-settings like ss01 or cv69? That is, even if terminal.integrated.fontLigatures is true, the font may not necessarily match the style sets and character variants enabled in the editor?

// settings.json

// Works
"terminal.integrated.fontLigatures": true

// Won't work as expected at this time?
"terminal.integrated.fontLigatures": "'calt', 'liga', 'ss19', 'cv69'"

Compare editorOptions.ts to terminalConfiguration.ts.

[Tyriar]

Cross posting from #34103 (comment)

If someone can figure out how to pass that information on to an OffscreenCanvas it should be trivial to add the support: https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvasRenderingContext2D#text_styles

[Tyriar]

I think we can support it if it weren't an OffscreenCanvas by setting the CSS. Which is a shame if that's the only way as we'd need to swap the canvas type depending on ligature config.

[Tyriar]

Relevant: https://github.com/xtermjs/xterm.js/blob/601efc3745c797ceaebe8707850fc071afafbd4e/addons/addon-ligatures/src/LigaturesAddon.ts#L39

[Tyriar]

Also: https://github.com/princjef/font-ligatures/blob/9963fd1bcd19d157202232e3da0b07cde511e49b/src/index.ts#L31

[Tyriar]

Upstream: xtermjs/xterm.js#5284