Add math tau (#7408)

# Description

Prompted by
https://discord.com/channels/601130461678272522/615329862395101194/1050693116501426216

# User-Facing Changes

New command `math tau` and endless agony whether to use `math pi` or
`math tau`

# Tests + Formatting

Example test

crates/nu-command/src/math/tau.rs

@@ -0,0 +1,61 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, IntoPipelineData, PipelineData, Signature, Type, Value};
+
+#[derive(Clone)]
+pub struct SubCommand;
+
+impl Command for SubCommand {
+    fn name(&self) -> &str {
+        "math tau"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("math tau")
+            .input_output_types(vec![(Type::Any, Type::Float)])
+            .category(Category::Math)
+    }
+
+    fn usage(&self) -> &str {
+        "Returns the mathematical constant τ."
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["trigonometry", "constant"]
+    }
+
+    #[allow(clippy::approx_constant)]
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            example: "(math tau) / 2",
+            description: "Compare π and τ",
+            result: Some(Value::test_float(std::f64::consts::PI)),
+        }]
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        Ok(Value::Float {
+            val: std::f64::consts::TAU,
+            span: call.head,
+        }
+        .into_pipeline_data())
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(SubCommand {})
+    }
+}