Add 1.1.8 to GitHub releases

[Marcono1234]

Brigadier 1.1.8 has been released as Maven artifact, but this GitHub repository still has 1.0.18 as latest release.
See https://libraries.minecraft.net/com/mojang/brigadier/1.1.8/brigadier-1.1.8.jar

Would also be good if you could explain the versioning scheme. The only code change was apparently #124 (and a JAR diff shows that only CommandDispatcher.class changed), so 1.0.18 → 1.1.8 does not seem to follow Semantic Versioning.