feat(editorconfig): add editorconfig syntax file

This is intentionally _not_ copied from Vim because our syntax file
makes use of Lua to dynamically generate a list of valid EditorConfig
properties. This requires the builtin editorconfig module, which Vim
does not have.

runtime/syntax/editorconfig.vim

@@ -0,0 +1,18 @@
+runtime! syntax/dosini.vim
+unlet! b:current_syntax
+
+syntax match editorconfigInvalidProperty "^\s*\zs\w\+\ze\s*="
+syntax keyword editorconfigProperty root
+
+lua<<
+local props = {}
+for k in pairs(require('editorconfig').properties) do
+  props[#props + 1] = k
+end
+vim.cmd(string.format('syntax keyword editorconfigProperty %s', table.concat(props, ' ')))
+.
+
+hi def link editorconfigInvalidProperty Error
+hi def link editorconfigProperty dosiniLabel
+
+let b:current_syntax = 'editorconfig'