add .editorconfig to set some basic coding styles (#225)

To maintain consistent code style between contributors and simplify merging, the solution should provide a .editorconfig to set some basic rules (already used by the current code source).

- use `UTF-8` with BOM (some files are without)
- use CR-LF
- use <kbd>Space</kbd> to indent (sorry #teamTab)
- trim trailing whitespace 
- be sure to add a new line at the end of files.

[Documentation](https://docs.microsoft.com/en-us/visualstudio/ide/create-portable-custom-editor-options?view=vs-2017)

src/.editorconfig

@@ -0,0 +1,11 @@
+root = true
+
+[*.{xaml,cpp,h}]
+charset = utf-8-bom
+indent_style = space
+indent_size = 4
+end_of_line = crlf
+
+[*.{cpp,h}]
+insert_final_newline = true
+trim_trailing_whitespace = true

src/Calculator.sln

@@ -9,6 +9,7 @@ Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "CalcManager", "CalcManager\
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{3A5DF651-B8A1-45CA-9135-964A6FC7F5D1}"
 	ProjectSection(SolutionItems) = preProject
+		.editorconfig = .editorconfig
 		nuget.config = nuget.config
 	EndProjectSection
 EndProject