Switch from tabs to spaces for indentation

The reason for this is mostly that the previous approach of with
tabstop=8 shiftwidth=4 isn't supported in modern editors.

Spaces solves this.

.editorconfig

@@ -0,0 +1,7 @@
+root = true
+
+[*.{c,h,y}]
+indent_size = 4
+indent_style = space
+trim_trailing_whitespace = true
+insert_final_newline = true

.gitattributes

@@ -0,0 +1,2 @@
+*.c     text diff=cpp whitespace=trailing-space,space-before-tab,tab-in-indent
+*.h     text diff=cpp whitespace=trailing-space,space-before-tab,tab-in-indent