add autocomplete for Config.LangKey annotation

[DarkEyeDragon]

Please include the following information in all issues:

• 2017.3-1.2.0
• 2017.3.5
• Windows

when filling in the
@Config.LangKey("some.key.")
there are no suggestions to autocomplete existing langkeys from the .lang files

Edit: this is for Forge btw.

[stale]

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