Add .editorconfig with basic rules for all files

This should make it easier to avoid whitespace errors, as long as the
editor used supports it (either natively or through a plugin).

See the editorconfig website for the editors that support it:

* https://editorconfig.org

Note: All text files appear to already be using LF and UTF-8 (or ASCII):

    $ git ls-files --eol | grep -v -e '^i/lf    w/lf' \
        -e 'i/none  w/none' -e 'i/-text w/-text'
    i/      w/      attr/text=auto eol=lf   ci/check/profiles/sort.py
    $ git ls-files -z | xargs -0 file -i -h | sed 's/[^:]*: *//' |
      grep -v -e 'charset=binary' -e 'charset=us-ascii' |
      LC_ALL=C sort | uniq -c
          1 text/html; charset=utf-8
          2 text/plain; charset=utf-8
          1 text/x-c; charset=utf-8

.editorconfig

@@ -0,0 +1,7 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true

.gitattributes

@@ -1 +1,3 @@
+* text=auto eol=lf
+
 /etc/inc/*.inc linguist-language=text