Simplify ligatures code

[tonsky]

Originally Fira Code uses scheme with spacer + single-space ligature that extends beyond its bounding box:

This was done to allow editors to “step inside” the ligature.

Since the original release, many editors now acknowledge the existence of ligatures and don’t need that trick. Maybe ligature code could be simplified?

Potential upsides:

• Much simple ligature code
• Ligatures could be assigned code points and used in e.g. Emacs Add glyphs to unicode reserved plane #211
• Fix ligatures inside RTL

Questions to check:

• Check which editors will not “step into” ligatures
• Check if Linux will be ok with double-, triple- etc size glyphs Fira Code v5 is not listed as monospace anymore under Linux #1073

[tonsky]

Another interesting behavior discovered in CoreText.

It feels like < is replaced with > in RTL first, then calt is applied, and then glyphs order is reversed for RTL text