into cellpath command (#7417)

# Description

Address part of feature request #7337, add a small command `into
cellpath` to allow string -> cellpath auto-conversion, with this change,
we could run

```
let p = 'ls.use_ls_colors'
$env.config | upsert ($p | nito cellpath) false
```

<img width="710" alt="image"
src="https://user-images.githubusercontent.com/85712372/206782818-3024b34f-150b-482d-aebc-9426ef6a1cf9.png">

Note - This pr only covers `String` -> `CellPath`, any other conversions
should be considered as expected?

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- [x] `cargo fmt --all -- --check` to check standard code formatting
(`cargo fmt --all` applies these changes)
- [x] `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- [x] `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

src/tests/test_env.rs

@@ -1,5 +1,7 @@
 use crate::tests::{run_test, TestResult};
 
+use super::run_test_with_default_config;
+
 #[test]
 fn shorthand_env_1() -> TestResult {
     run_test(r#"FOO=BAZ $env.FOO"#, "BAZ")
@@ -22,3 +24,11 @@ fn convert_non_string_env_var_to_nothing() -> TestResult {
         "nothing",
     )
 }
+
+#[test]
+fn convert_string_to_env_var_cellpath() -> TestResult {
+    run_test_with_default_config(
+        r#"let p = 'ls.use_ls_colors'; $env.config | upsert ($p | into cellpath) false | get ls.use_ls_colors"#,
+        "false",
+    )
+}