feat: initialize Geekeey Arcade .NET SDK repository

Single-commit squash of the repository history containing the Arcade
.NET MSBuild SDK packages (base, Razor, Web, Test), their analyzer and
editorconfig configuration, test suites, and CI workflows.

.editorconfig

@@ -0,0 +1,20 @@
+root = true
+
+[*]
+max_line_length = 120
+indent_style = tab
+indent_size = 2
+tab_width = 2
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+file_header_template = Copyright (c) The Geekeey Authors\nSPDX-License-Identifier: EUPL-1.2
+
+[*.md]
+indent_style = space
+trim_trailing_whitespace = false
+
+[*.yml]
+indent_style = space
+trim_trailing_whitespace = false