feat: update the nushell init file and make it valid module and overlay (#5188)

* break long commands into multiple lines for readability

* fix the format of closures

We generally write `{|| ...}` instead of `{ || ...}`.

* remove the `$"--opt=(val)"` structure when possible

`starship` does not require to use `--opt=val` and so we do not
need to do that with Nushell :)

the only place where this is required is with `--status` because
`$env.LAST_EXIT_CODE` can be negative and `starship` does not
appear to *like* values of the form `-2`...
so i left this line as it was.

on the other hand, `$env.CMD_DURATION_MS` and `(term size).columns`
should be fine 😌

* simplify the `config` mutation with new `?` syntax

This is a new very handy feature of Nushell which gives a much
simpler command combined with `default` and `merge`.

* put all `let-env`s inside an `export-env` with `load-env`

This commit has two reasons of existing:
- i think it makes it a bit easier to read with less `let-env`s
- it transforms the *script* into both a valid module and a valid
overlay

* bump the version to `0.78` in to docs

* add a note about the init file being also a module to all docs

* tweak the documentation

* update the Nushell part of the install script

* format the vuepress config file

as previous commit 117580136d was
not successful, let's try to make the CI happy manually 😌

* remove code quotes in the `config_cmd` of Nushell

* format the style in the Nushell `warning` section

docs/README.md

@@ -138,7 +138,7 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
    ::: warning
 
    This will change in the future.
-   Only Nushell v0.73+ is supported.
+   Only Nushell v0.78+ is supported.
 
    :::
 
@@ -151,8 +151,9 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
    And add the following to the end of your Nushell configuration (find it by running `$nu.config-path`):
 
    ```sh
-   source ~/.cache/starship/init.nu
+   use ~/.cache/starship/init.nu
    ```
+
    #### Xonsh
 
    Add the following to the end of `~/.xonshrc`: