Use heck instead of convert_case for nu-derive-value (#13708)

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# Description
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@sholderbach mentioned that I introduced `convert_case` as a dependency
while we already had `heck` for case conversion. So in this PR replaced
the use `convert_case` with `heck`. Mostly I rebuilt the `convert_case`
API with `heck` to work with it as I like the API of `convert_case` more
than `heck`.

# User-Facing Changes
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Nothing changed, the use of `convert_case` wasn't exposed anywhere and
all case conversions are still available.

# Tests + Formatting
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No new tests required but my tests in `test_derive` captured some errors
I made while developing this change, (hurray, tests work 🎉)
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

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crates/nu-derive-value/src/case.rs

@@ -0,0 +1,72 @@
+use heck::*;
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum Case {
+    // directly supported by heck
+    Pascal,
+    Camel,
+    Snake,
+    Kebab,
+    ScreamingSnake,
+    Title,
+    Cobol,
+    Train,
+
+    // custom variants
+    Upper,
+    Lower,
+    Flat,
+    ScreamingFlat,
+}
+
+impl Case {
+    pub fn from_str(s: impl AsRef<str>) -> Option<Self> {
+        match s.as_ref() {
+            // The matched case are all useful variants from `convert_case` with aliases
+            // that `serde` uses.
+            "PascalCase" | "UpperCamelCase" => Case::Pascal,
+            "camelCase" | "lowerCamelCase" => Case::Camel,
+            "snake_case" => Case::Snake,
+            "kebab-case" => Case::Kebab,
+            "SCREAMING_SNAKE_CASE" | "UPPER_SNAKE_CASE" | "SHOUTY_SNAKE_CASE" => {
+                Case::ScreamingSnake
+            }
+            "Title Case" => Case::Title,
+            "COBOL-CASE" | "SCREAMING-KEBAB-CASE" | "UPPER-KEBAB-CASE" => Case::Cobol,
+            "Train-Case" => Case::Train,
+
+            "UPPER CASE" | "UPPER WITH SPACES CASE" => Case::Upper,
+            "lower case" | "lower with spaces case" => Case::Lower,
+            "flatcase" | "lowercase" => Case::Flat,
+            "SCREAMINGFLATCASE" | "UPPERFLATCASE" | "UPPERCASE" => Case::ScreamingFlat,
+
+            _ => return None,
+        }
+        .into()
+    }
+}
+
+pub trait Casing {
+    fn to_case(&self, case: Case) -> String;
+}
+
+impl<T: AsRef<str>> Casing for T {
+    fn to_case(&self, case: Case) -> String {
+        let s = self.as_ref();
+        match case {
+            Case::Pascal => s.to_upper_camel_case(),
+            Case::Camel => s.to_lower_camel_case(),
+            Case::Snake => s.to_snake_case(),
+            Case::Kebab => s.to_kebab_case(),
+            Case::ScreamingSnake => s.to_shouty_snake_case(),
+            Case::Title => s.to_title_case(),
+            Case::Cobol => s.to_shouty_kebab_case(),
+            Case::Train => s.to_train_case(),
+
+            Case::Upper => s.to_shouty_snake_case().replace('_', " "),
+            Case::Lower => s.to_snake_case().replace('_', " "),
+            Case::Flat => s.to_snake_case().replace('_', ""),
+            Case::ScreamingFlat => s.to_shouty_snake_case().replace('_', ""),
+        }
+    }
+}