Re-enable the ability for users to define custom operators

[IwanKaramazow]

https://rescript-lang.org/blog/editor-support-custom-operators-and-more#custom-operators

[gaku-sei]

That's huge!! Thank you @IwanKaramazow 😄

[stale[bot]]

The rescript-lang/syntax repo is obsolete and will be archived soon. If this issue is still relevant, please reopen in the compiler repo (https://github.com/rescript-lang/rescript-compiler) or comment here to ask for it to be moved. Thank you for your contributions.