Add ADAPTER constant to KotlinLanguageAdapter

[awruff]

Description

Adds a constant to KotlinLanguageAdapter that'll allow you to easily reference the adapter in your Mod annotation.

// BEFORE
@Mod(
    modid = MyMod.ID,
    modLanguageAdapter = "org.polyfrost.oneconfig.utils.v1.forge.KotlinLanguageAdapter"
)

// AFTER
@Mod(
    modid = MyMod.ID,
    modLanguageAdapter = KotlinLanguageAdapter.ADAPTER
)

This allows mod devs to avoid needing to remember the class name. It also reduces the chance of typos

Related Issue(s)

N/A

Checklist

• I made a clear description of what was changed
• I stated why these changes were necessary
• I updated documentation or said what needs to be updated
• I made sure these changes are backwards compatible
• This pull request is for one feature/bug fix