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 :relieved:

* 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 117580136d3da5e00a4ccb7ada508c8510a2e50e was
not successful, let's try to make the CI happy manually :relieved:

* remove code quotes in the `config_cmd` of Nushell

* format the style in the Nushell `warning` section

1 files changed, 2 insertions, 2 deletions

diff --git a/README.md b/README.md
index b12c1a277..96fa1e000 100644
--- a/README.md
+++ b/README.md
@@ -346,10 +346,10 @@ starship init nu | save -f ~/.cache/starship/init.nu
 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
 ```
 
-Note: Only Nushell v0.73+ is supported
+Note: Only Nushell v0.78+ is supported
 
 </details>