CodeWriter rehaul to allow typed sections

[mtdowling]

This commit is a major refactor to CodeWriter to introduce a recursively typed AbstractCodeWriter that allows fluent interfaces to work with subtypes and allows the introduction of strongly typed section interceptors to CodeWriter.

This adds a major new feature to CodeWriter: typed sections and interceptors. This provides a much safer plugin system that can be used in code generators. Because the binding of an "interceptor" to a "section" is an interface in and of itself, code generation plugin systems don't need to actually hand off CodeWriters to plugins but rather just ask plugins for interceptors.

// Say you have your own AbstractCodeWriter subclass:
MyWriter writer = new MyWriter();

// Say you have an event named NameEvent that can be intercepted.
// The following lines register section interceptors for that event that just
// append to any previously written text in the section. The first interceptor
// uses one of the accessor methods of the provided NameEvent object.
writer.onSection(CodeInterceptor.appender(NameEvent.class, (w, section) -> w.write("$L", section.name())));
writer.onSection(CodeInterceptor.appender(NameEvent.class, (w, section) -> w.write("Foo")));
writer.onSection(CodeInterceptor.appender(NameEvent.class, (w, section) -> w.write("Bar")));

// injectSection is a shortcut for pushState(section).popState().
// popState() is when code interceptors are applied.
writer.injectSection(new NameEvent("Thomas"));

assert(writer.toString().equals("Name?\nThomas\nFoo\nBar\n"));

You can create a strongly typed section by passing a CodeSection to injectSection or pushState. A CodeSection is just an interface with a sectionName() method that returns the name of the class by default.

record NameEvent(String name) implements CodeSection;

NameEvent event = new NameEvent("Thomas");

You can create CodeInterceptors by implementing CodeInterceptor, or even better, CodeInterceptor.Appender / CodeInterceptor.Prepender that perform writing the previously emitted content correctly (only if non-empty and using writeInlineWithNoFormatting).

class SpecificNameInterceptor extends CodeInterceptor.Appender<NameEvent, MyWriter>() {
    private final String name;

    public SpecificNameInterceptor(String name) {
        this.name = name;
    }

    @Override
    public Class< NameEvent > sectionType() {
        return NameEvent.class;
    }

    @Override
    public void append(MyWriter writer, NameEvent section) {
        if (section.name().equals(name)) {
            writer.write("Hi, $L!", name);
        }
    }
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.