Report unreachable code warnings for if- and while-statements.

[Catalin-Andronie]

Issue: #136
Code review video: Episode 26

The IsError & IsWarning flags were added in the Diagnostic class to differentiate between warning and error diagnostics . Also the construction of a Diagnostic is now possible just via the Error and Warning static methods. Warnings are displayed in DarkYellow color.

• Block statements report unreachable code warning on the first statement span.
• No diagnostics are reported for empty block statements.
• Call expressions report warning on function name span (same on C#).

Warnings are reported for scenarios like:

    // Scenario 1
    if false
        continue // Report on [continue] span.

    // Scenario 2
    if false
        return
    else
        return // Report on [return] span.

    // Scenario 3
    while false
        print("") // Report on [print] span.

Until now I did NOT find a way to handle scenarios like:

    function test(): int
    {
        while true
        {
            return -1
        }

        // How to report warning here?
        // Can we use `ControlFlowGraph` when binding block statements??
        return 0
    }

Mate, you have amazing teaching skills 😄.
Your videos are better than Netflix. 👍

[ltrzesniewski]

I just wanted to point out a conflict with #132 due to a whitespace change. It's better to avoid formatting changes unrelated to the PR. 😉

Also, this should not conflict with #133, but it will need some adjustment to only bail out on errors. It will still work correctly after merging, but only because the lexer and parser never emit warnings.