Add support for font ligatures via xterm-addon-ligatures - Close #3607

[GitSquared]

This PR, which is possible thanks to the xTerm update in #3830, enables support for font ligatures thanks to xterm-addon-ligatures.

Note that this addon doesn't support the WebGL renderer (from xterm-addon-webgl) yet, but that renderer is considered unstable and was disabled during the upgrade to xTerm.js 4 anyway.

Hopefully by the time we get to re-enable the WebGL renderer ligatures will be supported on it. I added a comment to warn future maintainers (see line 141 in lib/components/term.js).

Related issues:
Close #3607

[GitSquared]

A little demo (Fira Code).

[Stanzilla]

Can you make this optional? There a lot of people that don't like ligatures, including me :)

[GitSquared]

@Stanzilla Wouldn't you simply use a font without programming ligatures if you don't want them?

So far the behaviour I've seen in other term emulators is that ligatures are automatically enabled if the font supports it - I'm not against the idea of implementing a boolean option but I think it will just confuse people trying to get ligatures in Hyper.

Just to clarify, only the handful of programming ligatures fonts like Fira Code and Iosevka are impacted by this PR; it doesn't change anything for "regular" fonts.

[Stanzilla]

@GitSquared is there a non-ligature version of Fira Code? I think it would just be good practice to add an option for it, VS Code has one as well.

[GitSquared]

@Stanzilla Yes, Fira Code is based on Mozilla's Fira. If we add an option, would you agree to make ligatures enabled by default?

[Stanzilla]

yeah would be fine by me

[GitSquared]

@Stanzilla I added a disableLigatures option, which defaults to false, and works as expected.

[Stanzilla]

Awesome, build only fails because of a linting issue

[GitSquared]

Should be fixed now 👍

[Stanzilla]

Great, thank you!