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/default_context.rs

@@ -419,6 +419,7 @@ pub fn create_default_context() -> EngineState {
             MathArcCosH,
             MathArcTanH,
             MathPi,
+            MathTau,
             MathEuler,
             MathLn,
         };