Update `.editorconfig` to match WordPress'

[netweb]

WordPress has updated .editorconfig file in ​https://core.trac.wordpress.org/changeset/42862/

Note: As part of this .jshintrc and *.json files now support tabbed indentation and will also be updated as part of this change

[DJPaul]

This is a low-value change overall, but if the code standards for indentation has changed in the JS files, please re-save them and commit the changes as well.

[djpaul]

In 11945:

Build: instruct file editors not to trim trailing whitespace in Markdown files.

Two trailing spaces in a markdown file indicates a line break.

BuddyPress does not contain Markdown files but this changes brings us inline with WordPress,
where historically we've matched code standards.

See ​#WP43104

Fixes #7733

Props netweb

[djpaul]

In 11946:

Build: instruct file editors to use tabs for indentation for .jshintrc and *.js files.

This changes brings us inline with WordPress, where historically we've matched code standards.

See ​#WP43075

Fixes #7733

Props netweb

[DJPaul]

We had no files that needed changing. Thanks for the patch @netweb :)

[netweb]

In 11947:

Build: Use tabs for indentaion, follow up to [11946]

See #7733.