Fixes #5284

This change does the following:

• Updates the fallbackLigatures to align with the upstream source
• Adds a fontFeatureSettings option to the ligatures addon which is passed on to the canvas
• Moves the webgl texture atlas canvas element to be a child of the terminal such that it inherits font-feature-settings from CSS, thereby enabling the correct ligature set and variants to kick in for webgl
• Recreates the webgl addon in the demo when ligatures is toggled. This is a little unfortunate that it's required at all, but other solutions to get the texture atlas updated immediately are more complicated

Fira Code ligatures disabled

Fira Code ligatures enabled

Fira Code ligatures enabled, fontFeatureSettings = "calt" on, "ss03" (see &)

Fira Code ligatures enabled, fontFeatureSettings = "calt" off, "ss03" (see &)