chore: Add basic EditorConfig

[hyperupcall]

It is difficult (annoying) to edit the shdoc file because there are trailing whitespace characters (a special option must be selected while saving, to save without formatting).

This fixes that by:

• Removing the trailing whitespace characters so others don't run into the issue
• Creating an .editorconfig, and configuring it to trim trailing whitespace characters so this doesn't happen in the future
  • I intentionally left indent_style and indent_size unconfigured