fs: ll_mkdir/write -> provider.mkdir/write

[XiaochenCui]

this is a task related to #1354

I moved the mkdir/write logic from ll_mkdir.js/ll_write.js to PuterFSProvider.js with minimal changes.

Test

This PR also passed all the apitest since it's based on #1376

[XiaochenCui]

hey @KernelDeimos
This PR is ready for review and merge, it also passed all the apitest since it's based on #1376

[KernelDeimos]

Did target_name have a different value here?

[XiaochenCui]

yeah, they are different is this request:

{
  "source": "//admin/api_test/move_cart_3/a/a_file.txt",
  "destination": "//admin/api_test/move_cart_3/b"
}
dest.get('name'): b
target_name: a_file.txt

and we are expecting b in the error:

puter/tools/api-tester/tests/move_cart.js

Lines 88 to 92 in 5b46f0b

	error_expected = {
	code: 'item_with_same_name_exists',
	message: 'An item with name `b` already exists.',
	entry_name: 'b',
	}

It's produced by:

node ./tools/api-tester/apitest.js --config=./tools/api-tester/config.yml --unit --suite=move_cart

[KernelDeimos]

Does /admin/api_test/move_cart_3/b already exist in this case?

[XiaochenCui]

yeah, exists as a normal file:

immutable =
0
is_dir =
0
is_public =
null
is_shortcut =
0
is_symlink =
0

It's created here:

puter/tools/api-tester/tests/move_cart.js

Lines 28 to 30 in 5b46f0b

	if ( state['conditions.destinationIsFile'] ) {
	await t.write(`${PREFIX}${i}/b`, 'placeholder\n');
	} else {