refactor(mrml-core): scope the dead_code allow to the render feature

[jdrouet]

mjml/mod.rs opens with a module-wide #![allow(dead_code)]. It turns out to be covering exactly two methods, MjmlAttributes::lang and ::dir, which are only reachable from mjml/render.rs and so look unused whenever the render feature is off.

Gating that impl block with #[cfg(feature = "render")] says the same thing precisely, and lets the blanket allow go. A module-wide allow silences the lint for everything else in the file too, permanently — which is how two methods end up hiding behind a switch that covers the whole module.

Verified with a clean compile under every relevant feature combination, since that is the only way this one shows up:

features	warnings
--no-default-features	0
+parse	0
+render	0
+json	0
+print	0
--all-features	0

Tests stay at 621.

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