Add line comment parsing

[bbugh]

Since mint-lang/mint#451 landed with line comments 🎉 🎉 🎉 🎉 🎉 🎉 this updates the vscode tmLanguage to support line comments. Works for both standalone and in-line:

// Hello! This is a comment. /* asdf */
fun doSomething (number : Number) { // Test comment 2 works
  "hello"
}

[gdotdesign]

Thanks :pray: