Add .indexes command

[yaziine]

List all indexes:

genji> .indexes

List indexes of a specific table:

genji> .indexes user

[tzzed]

The behaviour of .indexes command should only list indexes name or returns table index information.
What kind of output is expected? Output json document or Sql row?

[asdine]

Good question @tzzed, I think it should be the same as with the .tables command: Just a list of names, one per line

[yaziine]

I think adding the name of the table and the field whose the index is configured to may worth it.

genji> .indexes
idx_bar on user(name)
idx_foo on user(email)

where user is the table and name and email fields of that table.

[asdine]

I love this idea, let's do it ❤️

[tzzed]

Agree same as .tables.
What about the refactor of Index struct. Your suggestion leaves this issue out of context about the refactor because json.MarshalIndent(...) is not use.
So should I do an issue to suggest why?