refactor(encoding/csv): unified dir and move to single-export files

[timreichen]

ref: #2538

This PR moves all csv related stuff into the same dir.

• encoding/csv.ts → encoding/csv/mod.ts
• encoding/test_csv.ts → encoding/csv/mod_test.ts
• encoding/testdata/*.csv → encoding/csv/testdata/*.csv

splits logic into single export files.

⚠️ https://deno.land/std@$STD_VERSION/encoding/csv.ts is deprecated.

[iuioiua]

I don't think the original files with public exports should be deleted. Instead, a deprecation note should be added and the exported functions should come from the new function locations.

This gives devs enough time to migrate.

I did this here for the single-export restructure: https://github.com/denoland/deno_std/blob/7afe49284d0b5357eaf0f5668080cd26f13a1d8b/streams/buffer.ts

[iuioiua]

Also, it might be a good idea to also action #2936 at the same time, if possible.

[timreichen]

@iuioiua good points. I added the re-exports file and split parse and stringify into separate files.

[iuioiua]

For imports within examples, it might be better to link to the most direct file. For example:

import { parse } from "https://deno.land/std@$STD_VERSION/encoding/csv/parse.ts";

[iuioiua]

Related: #2936.

[timreichen]

closing in favour of #2936