Added an analyzer which keeps line breaks LF on every system (#1193)

app/SharedTools/Text/StringBuilderExtensions.cs

@@ -0,0 +1,45 @@
+using System.Runtime.CompilerServices;
+using System.Text;
+
+namespace SharedTools.Text;
+
+/// <summary>
+/// Appends lines with LF, the same line break on every system.
+/// </summary>
+/// <remarks>
+/// StringBuilder.AppendLine writes Environment.NewLine, which is CR LF on Windows: a text built with it differs between
+/// the systems, and it mixes two kinds of line breaks as soon as one of its parts comes with LF. These members carry a
+/// name of their own, because C# always prefers the instance method AppendLine over an extension of the same name.
+/// </remarks>
+public static class StringBuilderExtensions
+{
+    private const char LINE_FEED = '\n';
+
+    /// <param name="builder">The builder to append to.</param>
+    extension(StringBuilder builder)
+    {
+        /// <summary>
+        /// Appends a line break, LF.
+        /// </summary>
+        /// <returns>The builder, to append more.</returns>
+        public StringBuilder AppendLineLF() => builder.Append(LINE_FEED);
+
+        /// <summary>
+        /// Appends a text and a line break, LF.
+        /// </summary>
+        /// <param name="value">The text; for null, only the line break.</param>
+        /// <returns>The builder, to append more.</returns>
+        public StringBuilder AppendLineLF(string? value) => builder.Append(value).Append(LINE_FEED);
+
+        /// <summary>
+        /// Appends an interpolated text and a line break, LF.
+        /// </summary>
+        /// <remarks>
+        /// Like StringBuilder.AppendLine, the handler writes the text right into the builder, without a string in
+        /// between; afterward, only the line break is missing.
+        /// </remarks>
+        /// <param name="handler">The handler which the compiler creates for the interpolated text.</param>
+        /// <returns>The builder, to append more.</returns>
+        public StringBuilder AppendLineLF([InterpolatedStringHandlerArgument("builder")] ref StringBuilder.AppendInterpolatedStringHandler handler) => builder.Append(LINE_FEED);
+    }
+}