Standardise newline and trailing whitespace conventions

This change aims to reduce incidental changes in pull requests caused by
configuration and UI differences in text editors. The chosen conventions
are

- unix line endings
- trim trailing whitespace
- terminate files with a newline

editorconfig is an informal convention that indicates preferred line and
file terminations to text editors and other tools. Several editors have
builtin support, and many more have plugins available.

More info at https://editorconfig.org/

.editorconfig

@@ -0,0 +1,6 @@
+# https://editorconfig.org/
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_newspace = true