Ignore extension when renaming directory

[Limero]

I noticed when renaming a directory that had a . in the name that it would put the cursor before the dot. It doesn't make sense to treat that as a file extension, so this PR ignores this logic when renaming a directory.

[joelim-work]

Thanks for the changes, looks good to me.

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BTW, it might be useful to extract this as a utility function - unfortunately the standard library function filepath.Ext has the following problems:

• A hidden file (e.g. .gitignore) is treated as having an extension.
• The path is only evaluated as a string, so the result is the same regardless of whether it points to a file or a direectory.

The reason I mention this is that dupfilefmt (which you introduced in #1315) also has the same problem. But that can be addressed later.