Add TAU to approx_constant lint

[ghost]

Lint name: approx_constant

I tried this code:

let two_pi = 6.2833;

I expected to see this happen:
approx_const should recommend using TAU if my version setting is ≥ 1.47.

Instead, this happened:
Nothing. It's a false negative.

[rustbot]

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[ghost]

@rustbot assign @mikerite

[ghost]

Fixed by #7642