The `biome.json Directory Path` setting accepts file paths

[nhedger]

It appears that the biome.json Directory Path settings can be set to a file path, which causes resolution problems.

Using the file picker, we're restricted to directories, but it is still possible to manually enter a path that may not be a directory path.

An example of this issue can be seen in #4, where the user uses the /home/<...>/app/biome.json value for the setting, which the plugin considers to be a directory, and later attempts to find biome.json file at /home/<...>/app/biome.json/biome.json

We may want to ensure that the given path is a directory.