Port before_v0.60/config_management before_v0.60/language `before…

…_v0.60/prompt` `before_v0.60/tests` (nushell#851)

This PR is part of porting all old scripts nushell#221 and includes a set of
modules:

- `config_management`
- `language`
- `tests`
- `prompt`

## 12 files changed

### `config_management`
This module was removed, because it was related to the old `config.toml`
format
1. `README.md`: removed
2. `config.toml`: removed
3. `startup.nu`: removed

### `language`
4. `playground.nu` : unchanged because this is just `source ...`
5. `std.nu` : unchanged because this is just `source ...`
6. `playground/lib.nu` -> `playground/mod.nu` and all commands have been
exported
7. `std/date.nu` + `export` keyword

### `tests`
This module contained only the usage of `language` module, so I merged
it with `language`
8. `tests/language/std/date.nu` -> `modules/language/tests/date.nu`
9. `main.nu`: removed because it was just `source date.nu`

### `prompt`
10. `git_status_prompt.nu` -> `modules/git_status_prompt.nu` + `export`
keyword
11. `left_and_right_prompt.nu` -> `modules/left_and_right_prompt.nu` +
`export` keyword
12. `README.md` : has been removed as a duplicate of
`modules/prompt/README.md`