Improve DX around column default values and common raw SQL usage.

[igalklebanov]

Hey 👋

Temporal columns with a default value being current date and time are very common.
Forcing users to do:

import { sql } from 'kysely'

.addColumn('created_at', 'datetime', (cb) => cb.defaultTo(sql`CURRENT_TIMESTAMP`)) // MySQL / SQLite / PostgreSQL / MSSQL
// or
.addColumn('created_at', 'timestampz', (cb) => cb.defaultTo(sql`now()`)) // PostgreSQL

everywhere, is not ideal.

These also come up in issues/discord more often than desired.
There are also other defaultTo use cases that use raw SQL and are quite common.

Worth considering, but not the only way probably:

1. deprecate defaultTo.
2. introduce defaultLit for literals (strings, numbers, booleans, etc.).
3. introduce defaultRaw that accepts any raw SQL string, but has autocompletion for the common stuff like CURRENT_TIMESTAMP, NOW(), gen_random_uuid(), etc. no longer have to import sql template tag. Better DX.

Another approach, given there's not a lot of these:

1. introduce defaultToCurrentTimestamp().
2. introduce defaultToNow().