add `.env.nu` to load Nushell modules

adds a .env.nu file to load Nushell modules automatically thanks to the nuenv hook from the nu-hooks package.

.env.nu

+const MODULES = [
+    ".nushell/math.nu",
+    ".nushell/formats.nu",
+]
+
+def log-load [m: string] {
+    print $"[(ansi cyan_bold).env.nu(ansi reset)] loading (ansi purple)($m)(ansi reset)"
+}
+
+log-load $MODULES.0
+use $MODULES.0 *
+log-load $MODULES.1
+use $MODULES.1 *