feat: Add a .editorconfig.

[mparker17]

The Issue

(I didn't create an issue for this)

I was working on a shell script for for another addon request, and noticed that my editor's defaults didn't match ddev conventions. I went looking for a .editorconfig and didn't find one in the repo I was working on, so I tried adding one.

In a review, @stasadev asked me to update the template (i.e.: this repo) with an editorconfig; and update the UpdateChecker, and update the .gitattributes.

How This PR Solves The Issue

Adds an editorconfig. Adds a simple test for it in UpdateChecker. Adds the .editorconfig to the export-ignore list.

Manual Testing Instructions

1. Clone this branch
2. Create a new .sh file in the project using an editor that supports the EditorConfig standard with default configuration
3. Press the tab key:
   • Expected behavior: your cursor is indented 2 spaces
   • Previous behavior: your cursor is indented by something else (for me it was 4 spaces; but depending on the editor, it could be 8 spaces or 1 tab)

Automated Testing Overview

I updated the UpdateChecker test to look for the charset = utf-8 line, since that's probably a thing that should be in all ddev add-ons.

Release/Deployment Notes

None.

[stasadev]

Thank you!