Mysql string date support

[bherila]

Adds support to the mysql dialect to pass and parse dates as strings rather than js Date objects.

Installed locally.

bwh@Bens-Laptop bwh % yarn introspect              
yarn run v1.22.22
$ kysely-codegen --env-file=./.env.local --out-file=./src/server_lib/kysely.d.ts --date-parser=string
• Loaded environment variables from './.env.local'.
• No dialect specified. Assuming 'mysql'.
• Introspecting database...
✓ Introspected 37 tables and generated ./src/server_lib/kysely.d.ts in 213ms.

✨  Done in 0.73s.

Verified DATETIME columns are now generated like:

  when_added: Generated<string | null>;
  when_deleted: Generated<string | null>;

[RomanDronow]

I think you can already do this via "typeMapping" config property.