SQLite3 adapter supports expression indexes

[gkemmey]

@rafaelfranca I busted something in #33862 rebasing that closed the PR. Sorry about that. Didn't mean for this to become more trouble than it's worth 😅

Summary

Adds expression indexes support to SQLite adapter. Example:

create_table :users do |t|
  t.references :account
  t.string :email
end

add_index :users, 'account_id, lower(email)', name: 'index_users_on_account_id_and_email',
                                              unique: true