Add linter/formatter files

Added a .luacheckrc file to tell luacheck to set vim as a read-only
global.

Added .editorconfig to codify code style choices.

This allows users to hook into existing formatters and linters using
e.g. nvim-conform, nvim-lint.

Author: tomgeorge

.editorconfig

@@ -0,0 +1,8 @@
+[*]
+end_of_file = lf
+insert_final_newline = true
+
+[*.lua]
+indent_size = 2
+indent_style = space
+

.luacheckrc

@@ -0,0 +1,3 @@
+read_globals = {
+	"vim",
+}