Port before_v0.60/config_management before_v0.60/language before_v0.60/prompt before_v0.60/tests (#851)

This PR is part of porting all old scripts #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`

modules/language/playground/mod.nu

@@ -0,0 +1,54 @@
+export def main [topic, closure] {
+  with-env {N: 5 REJECT: slow } {
+    print (echo $topic " tests" (char newline) | str join)
+
+    do $closure
+  }
+}
+
+export def scene [
+  topic: any
+  --tag: string
+  closure: closure
+] {
+  print $"  ($topic)(char newline)"
+  do $closure
+}
+
+export def play [
+  topic: any
+  --tag: string
+  closure: closure
+] {
+  let is_tag_empty = ($tag | is-empty);
+  let should_run_all = ($env | get -i RUN_ALL | default false);
+
+  if $is_tag_empty {
+      do $closure $topic
+  } else {
+      if $tag == $env.REJECT and $should_run_all {
+          $"    ($topic) ... (ansi yellow)skipped(ansi reset) (char newline)"
+      } else {
+          do $closure $topic
+      }
+  }
+}
+
+export def expect [
+  topic: string
+  actual: list<any>
+  --to-be: list<any>
+] {
+  let are_equal = (($actual |  length) == ($to_be | length)) and ($actual | zip $to_be | all {|case|
+      $case.0 == $case.1
+    }
+ )
+
+  let line = (if true == $are_equal {
+          $"(ansi green)ok(ansi reset)(char newline)"
+      } else {
+          $"(ansi red)failed(ansi reset)(char newline)"
+      }
+  )
+  $"    ($topic) ... ($line)"
+}