Add .mts and .cts to TypeScript file extensions

See also:

https://github.com/github/linguist/pull/5905

CHANGELOG.md

@@ -23,6 +23,7 @@
 - Update `Command Help` syntax, see #2255
 - `Julia`: Fix syntax highlighting for function name starting with `struct`, see #2230
 - Minor update to `LiveScript`, see #2291
+- Associate `.mts` and `.cts` files with the `TypeScript` syntax. See #2236 (@kidonng)
 
 ## Themes