Add stack trace comment support to code writer

[mtdowling]

It's sometimes useful to know where in a code generator a line of code generated text came from. Enabling stack trace comments will output the last relevant stack trace information caused text to appear in the code writer's output.

MyCustomWriter writer = new MyCustomWriter();
writer.enableStackTraceComments(true); // <- this is new
writer.writeWithNoFormatting("Hello 1");
writer.write("Hello 2");
writer.indent().write("Hello 3. ");
writer.writeInline("Hello 4. ");
writer.writeInline("Hello 5. ");
System.out.println(writer.toString());

Outputs:

/* software.amazon.smithy.utils.CodeWriterTest.writesDebugInfoOnEachLine(CodeWriterTest.java:1367) */ Hello 1
/* software.amazon.smithy.utils.CodeWriterTest.writesDebugInfoOnEachLine(CodeWriterTest.java:1368) */ Hello 2
    /* software.amazon.smithy.utils.CodeWriterTest.writesDebugInfoOnEachLine(CodeWriterTest.java:1369) */ Hello 3. 
    /* software.amazon.smithy.utils.CodeWriterTest.writesDebugInfoOnEachLine(CodeWriterTest.java:1370) */ Hello 4. /* software.amazon.smithy.utils.CodeWriterTest.writesDebugInfoOnEachLine(CodeWriterTest.java:1371) */ Hello 5. 

The public API of this feature is just this method:

writer.enableStackTraceComments(true);

This tells the writer to turn on emitting stack trace comments. By default, the writer will emit C/Java-style block comments inline before calls to write, writeInline, writeWithNoFormatting, and writeInlineWithNoFormatting.

But not every language supports inline comments or C/Java-style block comments. So for example, in Python, you could do:

private static final class MyCustomWriter extends AbstractCodeWriter<MyCustomWriter> {
    @Override
    protected String formatWithStackTraceElement(String content, StackTraceElement element, boolean inline) {
        if (inline) {
            return content
        } else {
            return "# " + element + getNewline() + content;
        }
    }
}

The class will, by default, filter out any methods of AbstractCodeWriter and of the class that's extending it. Most implementations will not need to modify how frames are filtered, but this method is protected if you do:

@Override
protected boolean isStackTraceRelevant(StackTraceElement e) {
    return super.isStackTraceRelevant(e) && !e.getClassName().equals("Xyz");
}