Explicitly set tab_width = 8

Although tabs usually default to 8 columns, people who set their editors
to a different value for the tab width may see different indentation in
project files unless we explicitly specify a tab width of 8 in
`.editorconfig`.

Author: mhucka

.editorconfig

@@ -23,9 +23,10 @@ root = true
 charset = utf-8
 indent_style = space
 insert_final_newline = true
+max_line_length = 80
 spelling_language = en-US
+tab_width = 8
 trim_trailing_whitespace = true
-max_line_length = 80
 
 [{BUILD,*.BUILD,*.bzl,*.bazel,.bazelrc,WORKSPACE}]
 indent_size = 4