[CLI] Validate template files during merging

[peterjuras]

Summary

Files that are generated by coat are currently not being validated to ensure that files that are merged have the same properties such as the same file type, whether they are a local or global file or whether they are generated only once.

Currently the last file that is being merged dictates the file type and other properties. While this should work out for most cases, there might be issues where merging files throws errors due to mismatching file types.

Files should be validated while they are grouped to ensure that their properties are consistent and throw warnings or errors depending on the gravity of the property mismatch.