Add lean snippets and do Neovide

UltiSnips/lean.snippets

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+snippet ß "Backslash" A
+\\
+endsnippet
+
+snippet <> "Add sharp brackets"
+⟨$1⟩$0
+endsnippet
+
+# Mengen
+snippet N "Natural number"
+ℕ
+endsnippet
+
+snippet Z "Whole numbers"
+ℤ
+endsnippet
+
+
+# Quantoren
+snippet E "Exists"
+∃
+endsnippet
+
+snippet A "All"
+∀
+endsnippet