core!: move libxev and zig-aio event loops to USAGE.md

Create USAGE.md to serve as a friendly guide for usage.

Move the libxev and zig-aio event loops to serve as an example of how to
implement a custom event loop in USAGE.md. Remove dependency on these
libraries and all associated examples and source code.

The motivation for this change is to provide a simpler library. The
power to create a custom event loop will always be kept, however the
support burden of maintaining three event loops is not where I want to
focus my energy. The code is small enough that it's easy enough to be
copy/pasted into a project or provided as a third-party library.

build.zig

@@ -1,14 +1,10 @@
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
-    const include_libxev = b.option(bool, "libxev", "Enable support for libxev library (default: true)") orelse true;
     const include_images = b.option(bool, "images", "Enable support for images (default: true)") orelse true;
-    const include_aio = b.option(bool, "aio", "Enable support for zig-aio library (default: false)") orelse false;
 
     const options = b.addOptions();
-    options.addOption(bool, "libxev", include_libxev);
     options.addOption(bool, "images", include_images);
-    options.addOption(bool, "aio", include_aio);
 
     const options_mod = options.createModule();
 
@@ -25,14 +21,6 @@ pub fn build(b: *std.Build) void {
         .optimize = optimize,
         .target = target,
     }) else null;
-    const xev_dep = if (include_libxev) b.lazyDependency("libxev", .{
-        .optimize = optimize,
-        .target = target,
-    }) else null;
-    const aio_dep = if (include_aio) b.lazyDependency("aio", .{
-        .optimize = optimize,
-        .target = target,
-    }) else null;
 
     // Module
     const vaxis_mod = b.addModule("vaxis", .{
@@ -44,9 +32,6 @@ pub fn build(b: *std.Build) void {
     vaxis_mod.addImport("grapheme", zg_dep.module("grapheme"));
     vaxis_mod.addImport("DisplayWidth", zg_dep.module("DisplayWidth"));
     if (zigimg_dep) |dep| vaxis_mod.addImport("zigimg", dep.module("zigimg"));
-    if (xev_dep) |dep| vaxis_mod.addImport("xev", dep.module("xev"));
-    if (aio_dep) |dep| vaxis_mod.addImport("aio", dep.module("aio"));
-    if (aio_dep) |dep| vaxis_mod.addImport("coro", dep.module("coro"));
     vaxis_mod.addImport("build_options", options_mod);
 
     // Examples
@@ -75,9 +60,6 @@ pub fn build(b: *std.Build) void {
         .optimize = optimize,
     });
     example.root_module.addImport("vaxis", vaxis_mod);
-    if (xev_dep) |dep| example.root_module.addImport("xev", dep.module("xev"));
-    if (aio_dep) |dep| example.root_module.addImport("aio", dep.module("aio"));
-    if (aio_dep) |dep| example.root_module.addImport("coro", dep.module("coro"));
 
     const example_run = b.addRunArtifact(example);
     example_step.dependOn(&example_run.step);

build.zig.zon

@@ -12,16 +12,6 @@
             .url = "git+https://codeberg.org/dude_the_builder/zg#7ddce488e074c3f052949ba513a340446cea86e9",
             .hash = "12205e2e7108385a1ec280b6365166e902a690d6a0e8fcc015fdea62d63cf79be8ec",
         },
-        .libxev = .{
-            .url = "git+https://github.com/mitchellh/libxev#f6a672a78436d8efee1aa847a43a900ad773618b",
-            .hash = "12207b7a5b538ffb7fb18f954ae17d2f8490b6e3778a9e30564ad82c58ee8da52361",
-            .lazy = true,
-        },
-        .aio = .{
-            .url = "git+https://github.com/Cloudef/zig-aio#b5a407344379508466c5dcbe4c74438a6166e2ca",
-            .hash = "1220a55aedabdd10578d0c514719ea39ae1bc6d7ed990f508dc100db7f0ccf391437",
-            .lazy = true,
-        },
     },
     .paths = .{
         "LICENSE",