[META][DX][BC] change db_add_field() to db_add_column()

[jenlampton]

We have a DX problem with db_add_field() in that the term "field" is confusing. We have fields on entities, and fields in forms, but "field" is not a common term in databases, particularly relational databases like MySQL.

We inherited db_add_field() and friends (db_drop_field(), db_change_field(), db_escape_field(), db_field_names(), db_field_exists(), db_field_set_default(), db_field_set_no_default()) from Drupal 7. Let's deprecate the lot of them, and make equivalents using the word "column" instead.

• [ ] @todo db_add_field()
• [ ] @todo db_drop_field()
• [ ] @todo db_change_field()
• [ ] @todo db_escape_field()
• [ ] @todo db_field_names()
• [ ] @todo db_field_exists()
• [ ] @todo db_field_set_default()
• [ ] @todo db_field_set_no_default()
• [ ] @todo db_update()->fields()
• [ ] @todo db_select->fields()

[quicksketch]

The word "field" is also used in a few methods, such as db_update()->fields() and db_select->fields().

This change definitely makes sense in my mind, but it's also a fair amount of API change. Clearly we can help ease the transition with a BC layer (just have all "field" functions/methods call "column" ones). If we take that approach, we could remove BC in 2.x and implement it in 1.x whenever.