Antoine Stevan 01d8961eb7
use to_lowercase in str downcase (#10850)
# Description
as we can see in the [documentation of
`str.to_lowercase`](https://doc.rust-lang.org/std/primitive.str.html#method.to_lowercase),
not only ASCII symbols have lower and upper variants.

- `str upcase` uses the correct method to convert the string

7ac5a01e2f/crates/nu-command/src/strings/str_/case/upcase.rs (L93)
- `str downcase` incorrectly converts only ASCII characters

7ac5a01e2f/crates/nu-command/src/strings/str_/case/downcase.rs (L124)

this PR uses `str.to_lower_case` instead of `str.to_ascii_lowercase` in
`str downcase`.

# User-Facing Changes
- upcase still works fine
```nushell
~ l> "ὀδυσσεύς" | str upcase
ὈΔΥΣΣΕΎΣ
```
- downcase now works

👉 before
```nushell
~ l> "ὈΔΥΣΣΕΎΣ" | str downcase
ὈΔΥΣΣΕΎΣ
```
👉 after
```nushell
~ l> "ὈΔΥΣΣΕΎΣ" | str downcase
ὀδυσσεύς
```

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
-  `toolkit test`
-  `toolkit test stdlib`

adds two tests
- `non_ascii_upcase`
- `non_ascii_downcase`

# After Submitting
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