feat: Adds some documentation strings in ManualDCore module.

Sources/ManualDCore/Extensions/Numbers+string.swift

@@ -0,0 +1,24 @@
+import Foundation
+
+extension Double {
+
+  public func string(digits: Int = 2) -> String {
+    numberString(self, digits: digits)
+  }
+}
+
+extension Int {
+
+  public func string() -> String {
+    numberString(Double(self), digits: 0)
+  }
+}
+
+private func numberString(_ value: Double, digits: Int = 2) -> String {
+  let formatter = NumberFormatter()
+  formatter.maximumFractionDigits = digits
+  formatter.groupingSize = 3
+  formatter.groupingSeparator = ","
+  formatter.numberStyle = .decimal
+  return formatter.string(for: value)!
+}