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/prompt/git_status_prompt.nu

@@ -0,0 +1,97 @@
+# Displays a prompt
+export def git-status-prompt [] {
+  let not_windows = ($nu.os-info.name !~ "windows")
+  $"(ansi reset)(ansi green)(if $not_windows {
+      $env.USER
+  } else {
+      $env.USERNAME
+  }
+  )(ansi reset)@(hostname | str trim):(ansi green_dimmed)(prompt-pwd)(ansi reset)(git-branch-icon)(ansi reset)(char newline)(char prompt) "
+}
+
+# Returns a shortened pwd for use in prompt
+def prompt-pwd [] {
+  let not_windows = ($nu.os-info.name !~ "windows")
+  let path = (pwd | if $not_windows { split row "/" } else { split row '\' })
+  let home = (if $not_windows {
+      ($env.HOME | split row "/")
+  } else {
+      (echo [$env.HOMEDRIVE $env.HOMEPATH] | path join | split row '\')
+  }
+  )
+
+  if ($path | length) > 1 {
+      if ($home | all {|it| $it in $path }) {
+          let path_without_home = ($path | skip ($home | length))
+          if ($path_without_home | wrap path | compact | length) > 0 {
+            let parent = ($path | skip ($home | length) | drop)
+            if ($parent | wrap parent | compact | length) > 0 {
+                let short_part = ($parent | each { |part|
+                                  if ($part | str starts-with ".") {
+                                      $"($part | str substring [0 2])/"
+                                  } else {
+                                      $"($part | str substring [0 1])/"
+                                  }
+                })
+                $"~/( | str join)($path | last)"
+            } else {
+                $"~/($path | last)"
+            }
+        } else {
+            "~"
+          }
+      } else {
+          let parent = (echo $path | drop | str substring [0 1] | each {|it| echo $it "/" })
+          $"/($parent)($path | last)"
+      }
+  } else {
+      pwd
+  }
+  }
+
+# Map of git status codes to ANSI colour codes
+def git-prompt-map [] {
+  echo a m r c d "??" u |
+  rotate --ccw |
+  reject column0 | append (
+    echo (ansi green) (ansi yellow_bold) (ansi cyan) (ansi blue) (ansi red) (ansi red_dimmed) (ansi red) |
+    rotate --ccw |
+    reject column0
+  ) | headers
+}
+
+# Gets an icon and a colour for a given git status code
+def git-prompt-icons [k] {
+  let icns = ["✚ " "* " "➜ " "⇒ " "✖ " "? " "! "];
+
+  git-prompt-map |
+  transpose status colour | enumerate | each { |icon|
+        let idx = $icon.index;
+        if $icon.item.status == $k {
+            $"($icon.item.colour)($icns | get $idx)"
+        }
+  } | compact
+}
+
+# Checks git status of current working directory and displays an icon
+def git-branch-icon [] {
+  do -i {
+    let branch = (do -i { git rev-parse --abbrev-ref HEAD } | str trim)
+
+    if ($branch | str length) > 0 {
+        let modified = (do -i { git status --porcelain } | split row "\n" | str trim | split column " " status file);
+        if ($modified | get 0 | first | is-empty) {
+            $"|(ansi green)($branch)(ansi reset):(ansi green)✓(ansi reset)"
+        } else {
+            let modified2 = (do -i { git status --porcelain } | split row "\n" | str substring [0 1])
+            let branch_colour = (if (echo $modified2 | each {|it| $it in [A M R C D] } | reduce {|it acc| $it or $acc }) {
+                    "yellow"
+                } else {
+                    "red"
+                }
+            )
+            $"|(ansi $branch_colour)($branch)(ansi reset):($modified | get status | uniq | str downcase | each {|it| git-prompt-icons $it })" | str join
+        }
+    }
+  }
+}