More granular options to ss01 and alternatives for (== and != and === and !==)

The `ss01` option includes these ligatures seemingly inspired by Fira Code's `liga`:
- `==` becomes a wide equal sign with no visible gap
- `!=` adds a slash across this `==`
- `===` becomes a stack of three dashes
- `!==` adds a slash across this `===`

I was never fond of this, as it makes `=` and `==` less distinguishable and obfuscates the meaning of `===`. Thankfully, Fira Code offers an alternative via `ss08`:
- `==` merges somewhat but retains a small gap
- `!=` adds a slash across this `==` but the gap remains noticeable
- `===` merges somewhat but retains small gaps
- `!==` adds a slash across the middle `=` of this `===`

It would be nice if these were also available in Monaspace.

That said, this draws attention to a bigger issue: `ss01` includes many unrelated ligatures that you wouldn't want enabled in all contexts. For instance, the substitutions for `!~`, `~~`, `=~` and `~~` would make no sense in languages where `~` is the binary inverse operator, yet you can't leave them out if you want to enable the substitutions for `!=` and `!==`.

まず、issue に一覧表示されている既存の ss01 置換を確認し、Fira Code の ss08 の動作と比較します。==、!=、===、!== の代替表現を、関連のない !~、~~、=~ の置換とは独立して公開する方法を検討します。要求された代替表現と、より細 granular な選択が、関連のない合字を強制せずに Monaspace で利用できるようになれば完了です。

索引モデルが issue の本文から書いたものです。