Add stylistic set for Typst compatibility

Hi! I'm developing a Typst editor and would love to use Fira Code, but a couple of ligatures conflicts with Typst's syntax. (Typst is a modern markup-based typesetting system in the vein of LaTeX.)

Issues:

1. Heading syntax with ===

Typst uses = signs for headings:

= Heading - Level 1
== Heading - Level 2
=== Heading - Level 3
==== Heading - Level 4

In stylistic set ss08, the === ligature adds a third horizontal bar, making it visually distinct from = and ==. This breaks the visual consistency of heading syntax—all heading markers should look like repeated equals signs, not transform into a different glyph.

2. En-dashes

Typst renders -- as an en-dash (–), which is standard typographic convention. However, Fira Code's -- ligature renders as a programming-style glyph with a small break in the middle.

Would it be possible to add a stylistic set optimized for Typst that adjusts the === ligature and renders -- as a continuous en-dash instead of a code ligature? This would make Fira Code perfect for markup/typesetting languages like Typst that mix code and typography.

Thanks for maintaining such an excellent font!

Review Fira Code's existing stylistic-set behavior, especially ss08, and the handling of the === and -- ligatures. The work is done when a Typst-oriented stylistic set keeps repeated equals signs visually consistent and renders -- as a continuous en-dash without changing the default behavior.