Add Markdown trailing whitespace rule to `.editorconfig`

[netweb]

With Gutenberg being merged as part of WordPress 5.0 having WordPress' build tools have parity with that of Gutenberg will be advantageous and one less issue to worry about when merge time comes.

This ticket adds support to .editorconfig to not trim trailing whitespace

See ​https://github.com/WordPress/gutenberg/blob/master/.editorconfig#L20-L21

[SergeyBiryukov]

Added in ​https://github.com/WordPress/gutenberg/commit/906bbab2e157d953bd83abcd640899deffd83ef3.

Given that WordPress core does not have any .md files, not sure how necessary this is, though don't see any harm either :)

[netweb]

Replying to SergeyBiryukov:

Added in ​https://github.com/WordPress/gutenberg/commit/906bbab2e157d953bd83abcd640899deffd83ef3.

Given that WordPress core does not have any .md files, not sure how necessary this is, though don't see any harm either :)

I was being preemptive as Gutenberg includes lots of Markdown files and will be merged as part of WP 5.0 it is one task taken care of now rather than later at merge time 😏

[johnbillion]

This also affects markdown files that you're editing that aren't part of core, for example a readme.md file that lives inside a plugin or theme. The rules for .editorconfig evidently bubble up in some editors.

[johnbillion]

In 42862:

General: Instruct file editors not to trim trailing whitespace in markdown files.

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

Props netweb

Fixes #43104

[johnbillion]

In 43693:

General: Instruct file editors not to trim trailing whitespace in markdown files.

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

Props netweb

Fixes #43104

Merges [42862] to the 5.0 branch.