CHORE: add .editorconfig file

Documentation:
https://editorconfig.org/

Refs: #17

.editorconfig

+# This file is for unifying the coding style
+# for different editors and IDEs.
+#
+# More information at http://editorconfig.org/
+##############################################
+
+root = true
+
+##############################################
+#      default charset
+#    + 4 space indentation
+#    + Unix-style newlines
+#    + remove any whitespace characters preceding newline characters
+##############################################
+[*]
+charset      = utf-8
+indent_style = space
+indent_size  = 4
+end_of_line  = lf
+insert_final_newline     = true
+trim_trailing_whitespace = true
+
+[*.ts]
+indent_style = tab
+