feat: add expo config plugin for mmkv log level

[bryanprimus]

adds expo config plugin support for configuring mmkv native log level during prebuild

mmkv already supports build time log level configuration through

android:

MMKV_logLevel in android/gradle.properties

iOS:

$MMKVLogLevel in ios/Podfile

#995

expo users should not manually edit generated native files, so this exposes the existing native configuration through the package config plugin

[
  "react-native-mmkv",
  {
    "logLevel": 4
  }
]