Convert FluentSyntax.jsm to an ES module

[gregtatum]

From Bug 1834205:

As part of the ongoing work, we should convert FluentSyntax.jsm to an ES module.

https://github.com/projectfluent/fluent.js/blob/f877b2920efeb82e33cde38b219c5d59df853ca8/fluent-gecko/src/fluent-syntax.js

fluent.js/fluent-gecko/rollup.config.mjs

Line 45 in f877b29

file: "dist/FluentSyntax.jsm",