EditorConfig: Support spelling_language (validation only) #43

This feature is needed für support
of the spelling_language EditorConfig property

The spelling_language is the first property where ecformat only can
validate the set value itself and not that files respect it.
For this is makes no sense to validate property for every file
where it applies to.

I reported the bug regarding the handling of unset in this situation.
Without this workaround a "ecformat check" fails on the
ecformat repository itself due to the "spelling_language = unset"
in LICENSES/.editorconfig which unsets all properties. Even for
properties which are currently not set in the root .editorconfig.

As the ignore crate does not consider which files are actually in the
repository, ignoring them with a gitignore implies that ecformat ignores
them while validating the .editorconfig files.

To allow usage in tests for get_editorconfig_files

Although ignoring all hidden files is not acceptable at the search for
.editorconfig files as they are hidden themself, hidden directories
have to be ignored to work as documented.

This avoids that the automatic changes in this directory conflict
with the EditorConfig for these files. Furthermore, its a nice test case
for the get_editorconfig_files function.

The get_target_files function never returns an error as it returns
them related to single files inside the iterator, the same way as
get_editorconfig_files does.

Disabling the repeat rule as repeating the first word of the sentence
across multiple comments is not an issue.

As reminder to remove them when upgrading to the ec4rs version
that contains the bug fixes.