add support for custom theming in mers errors

this also includes support for the NoTheme,
a theme which doesn't add any color to mers'
output.
If you compile mers with --no-default-features,
the `colored` dependency will disappear and
mers_lib will fall back to NoTheme.

mers_lib/examples/00_parse_compile_check_run.rs

@@ -24,7 +24,7 @@ fn show(src: String) {
     );
     eprintln!("{src}");
     match parse_compile_check_run(src) {
-        Err(e) => eprintln!("{e}"),
+        Err(e) => eprintln!("{e:?}"),
         Ok((t, v)) => eprintln!("Returned `{}` :: `{t}`", v.get()),
     }
 }

mers_lib/examples/02_own_variable.rs

@@ -14,7 +14,7 @@ fn main() {
 
     eprintln!("This is not:");
     let e = run("5.rust_func".to_owned()).err().unwrap();
-    eprintln!("{e}");
+    eprintln!("{e:?}");
 }
 fn run(src: String) -> Result<(), CheckError> {
     let mut source = Source::new_from_string(src);