Add support for passing null, string literals, enum lists and more to b.dependency()

[castholm]

I was getting pretty fed up with not being able to pass null values as options to dependencies, which prevents this relatively common use case from working:

const target = b.standardTargetOptions(.{});
const optimize = b.standardOptimizeOption(.{});

// Optional option; if null, use some sane default
const strip = b.option(bool, "strip", "Strip debug symbols");

const dep = b.dependency("foo", .{
    .target = target,
    .optimize = optimize,
    .strip = strip, // compile error!
});

The only current workaround to this problem is to use a separate b.dependency() invocation for the null case, which is annoying (and the number of separate invocations rapidly increases the more optional options there are!).

This PR first adds a fairly comprehensive standalone test case that tests the behavior of b.dependency(). It then implements support for passing

• null (as literals or as explicit optionals)
  • passing null is the same as not specifying the option
• string literals
• values coercible to []const T (including strings and string/lazy path lists)
• enum lists
• std.zig.BuildId

to b.dependency() and updates the test accordingly. See test/standalone/dependency_options/build.zig for a demonstration of the new types of option values these changes allow for.

Closes #21609
Closes #21928
(I only encountered these after I started working on this branch. Sorry!)