Refactor language config loading

[sudormrfbin]

I'm currently implementing #976 and had to access the language configuration in the same way that the editor normally does. This PR factors out the relavant code to functions defined in helix-core.

[colemickens]

When building with Nix, I now get a failure:

rust_helix-core> error: aborting due to previous error                            
error: builder for '/nix/store/6lx3j3lllys6h0s1q7sgcz0h2pmwx3yj-rust_helix-core-0.
6.0.drv' failed with exit code 1;                                                 
       last 10 log lines:                                                         
       > error: couldn't read src/../../languages.toml: No such file or directory 
(os error 2)                                                                      
       >  --> src/config.rs:5:22                                                  
       >   |                                                                      
       > 5 |     toml::from_slice(include_bytes!("../../languages.toml"))         
       >   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^          
       >   |                                                                      
       >   = note: this error originates in the macro `include_bytes` (in Nightly 
builds, run with -Z macro-backtrace for more info)