Add a way for consumers of sokol-zig to install the Emscripten SDK

[Katipo007]

As of the changes in #156 consumers of the sokol-zig library no longer have a direct way of installing the Emscripten SDK from their own zig build.

In the mentioned pull request, the install of the SDK is now performed via zig build install-emsdk step, and (as far as I'm aware) there is no way for a downstream build.zig to perform or depend upon an upstream step.

The changes in this pull request expose the function which creates a std.Build.Step to perform the install and activation of the SDK so that downstream projects have a way to perform the install as they see fit.

e.g.

// build.zig
const std = @import("std");
pub fn build(b: *std.Build) void {
    const sokol_dep = b.dependency("sokol", .{});
    const emsdk_dep = sokol_dep.builder.dependency("emsdk", .{});

    // Create a step to handle installing the Emscripten SDK
    const emsdk_install_step = @import("sokol").emSdkInstallStep(b, emsdk_dep, .{});

    // We can now expose our for manual install:
    b.step("install-emsdk", "Install Emscripten SDK in zig-pkg").dependOn(emsdk_install_step);

    // Or add it as a dependency to the "sokol_clib" artifact so that it is performed automatically
    sokol_dep.artifact("sokol_clib").step.dependOn(emsdk_install_step);
}

I have further ideas on how to streamline this process further, but it is conditional on a feature such as https://codeberg.org/ziglang/zig/issues/35614 being implemented to Zig, and thus would not be backwards compatible with 0.16.0

[floooh]

Good fix, thanks! I'll give it a whirl later today and then merge.

[floooh]

Ok tested and merged. Thanks!