to <format>: preserve round float numbers' type (#16016)

- fixes #16011

# Description
`Display` implementation for `f64` omits the decimal part for round
numbers, and by using it we did the same.
This affected:
- conversions to delimited formats: `csv`, `tsv`
- textual formats: `html`, `md`, `text`
- pretty printed `json` (`--raw` was unaffected)
- how single float values are displayed in the REPL

> [!TIP]
> This PR fixes our existing json pretty printing implementation.
> We can likely switch to using serde_json's impl using its
PrettyFormatter which allows arbitrary indent strings.

# User-Facing Changes
- Round trips through `csv`, `tsv`, and `json` preserve the type of
round floats.
- It's always clear whether a number is an integer or a float in the
REPL
  ```nushell
  4 / 2
  # => 2  # before: is this an int or a float?

  4 / 2
  # => 2.0  # after: clearly a float
  ``` 

# Tests + Formatting
Adjusted tests for the new behavior.

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-utils/src/float.rs

@@ -0,0 +1,21 @@
+use std::fmt::{Display, LowerExp};
+
+/// A f64 wrapper that formats whole numbers with a decimal point.
+pub struct ObviousFloat(pub f64);
+
+impl Display for ObviousFloat {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let val = self.0;
+        if val.fract() == 0.0 {
+            write!(f, "{val:.1}")
+        } else {
+            Display::fmt(&val, f)
+        }
+    }
+}
+
+impl LowerExp for ObviousFloat {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        LowerExp::fmt(&self.0, f)
+    }
+}

crates/nu-utils/src/lib.rs

@@ -4,6 +4,7 @@ mod deansi;
 pub mod emoji;
 pub mod filesystem;
 pub mod flatten_json;
+pub mod float;
 pub mod locale;
 mod quoting;
 mod shared_cow;
@@ -22,6 +23,7 @@ pub use deansi::{
 };
 pub use emoji::contains_emoji;
 pub use flatten_json::JsonFlattener;
+pub use float::ObviousFloat;
 pub use quoting::{escape_quote_string, needs_quoting};
 pub use shared_cow::SharedCow;