should support filetype detection when none is available

[mitermayer]

We should provide some basic filetype detection but also keeping compatible with other plugins.

This for example would enable to correctly identify graphql, typescript, less, scss etc.. without having to rely on the filetype being implemented by other plugins.

we should be able to leverage from vim did_filetype() function that will allow us to conditionally set a filetype if no other plugin has done so.