Do not set indent variable if `indent_size` is not in the output from `editorconfig`.
More indentation settings.
Allow derived mode to use parent mode setting.

Allow derived mode to use the setting of the closest parent mode which
exists in `edconf-indentation-alist`.

E.g., `doctex-mode` doesn't have to add its own setting into the alist.
Instead, it can use the setting of `latex-mode` if they use the same
indent variables.