Cop idea: Prefer symbol keys over string keys in migrations

[ydakuka]

Add a cop that enforces the use of symbol arguments instead of string arguments in migration DSL methods:

# bad
t.string 'uuid', limit: 36, null: false, index: { unique: true }
t.references 'language', type: :integer, foreign_key: true, null: false

# good
t.string :uuid, limit: 36, null: false, index: { unique: true }
t.references :language, type: :integer, foreign_key: true, null: false