AddClause does not indent the clauses of an interface implementation

[petithug]

When using AddClause on an interface implementation, the clause should be indented.

Steps to Reproduce

Loading the following in the REPL and executing ":ac 4 Show"

data Blah : Type where
  MkBlah : Blah

Show Blah where

Expected Behavior

data Blah : Type where
  MkBlah : Blah

Show Blah where
  showPrec d x = ?showPrec_rhs

Observed Behavior

data Blah : Type where
  MkBlah : Blah

Show Blah where
showPrec d x = ?showPrec_rhs