`build.zig`: Make optional `fetch` dependencies

[kassane]

Based on #42 (comment)

Currently, build.zig/build.zig.zon adds some dependencies that will fetch all the complementary tools and libraries (even if you don't use them).
The packages listed in the build.zig.zon are intentionally downloaded, even though there is a lazyDepedency.

cc: @floooh

References

• Associate lazy dependencies with steps and only fetch them if those steps are built ziglang/zig#21525
• Transitive lazy dependencies are not fetched ziglang/zig#20716
• Implement system package mode ziglang/zig#18778

[kassane]

initial test with LazyDependency. Now, only imgui is optional dep.
commit: 3332ca6