fix: support mmdc on windows (#319)

I am not sure if Windows platform support is of any importance but I
encountered an issue where `mmdc` cannot be found on Windows. This is
due to this
[issue](https://doc.rust-lang.org/std/process/struct.Command.html#implementations)
where `Command::new()` on Windows works in this way: "if the file has a
different extension, a filename including the extension needs to be
provided, otherwise the file won’t be found.".

Since mmdc on Windows is an alias for the `mmdc.cmd` script and not an
`exe`, this updates the name used to find the mmdc tool.

src/tools.rs

@@ -18,7 +18,8 @@ impl ThirdPartyTools {
     }
 
     pub(crate) fn mermaid(args: &[&str]) -> Tool {
-        Tool::new("mmdc", args)
+        let mmdc = if cfg!(windows) { "mmdc.cmd" } else { "mmdc" };
+        Tool::new(mmdc, args)
     }
 
     pub(crate) fn presenterm_export(args: &[&str]) -> Tool {