Part of #27065
This PR touches functions used in templates. As templates are not static
typed, errors are harder to find, but I hope I catch it all. I think
some tests from other persons do not hurt.
There are 2 kinds of ".Editorconfig" in code, one is `JSON string` for
the web edtior, another is `*editorconfig.Editorconfig` for the file
rendering (used by `TabSizeClass`)
This PR distinguish them with different names.
And by the way, change the default tab size from 8 to 4, I think few
people would like to use 8-size tabs nowadays.