feat(build): build.zig MVP: build and run functionaltests on linux

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

deps/utf8proc/build.zig

@@ -0,0 +1,24 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) !void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    const upstream = b.dependency("utf8proc", .{});
+    const lib = b.addStaticLibrary(.{
+        .name = "utf8proc",
+        .target = target,
+        .optimize = optimize,
+    });
+
+    lib.addIncludePath(upstream.path(""));
+    lib.installHeader(upstream.path("utf8proc.h"), "utf8proc.h");
+
+    lib.linkLibC();
+
+    lib.addCSourceFiles(.{ .root = upstream.path(""), .files = &.{
+        "utf8proc.c",
+    } });
+
+    b.installArtifact(lib);
+}

deps/utf8proc/build.zig.zon

@@ -0,0 +1,12 @@
+.{
+    .name = "utf8proc",
+    .version = "2.10.0",
+    .paths = .{""},
+
+    .dependencies = .{
+        .utf8proc = .{
+            .url = "git+https://github.com/JuliaStrings/utf8proc?ref=v2.10.0#a1b99daa2a3393884220264c927a48ba1251a9c6",
+            .hash = "1220d80ce0bde71b8acae76b9686fa785aa0cdf165f0d6e93a853d57bfbed129a5e7",
+        },
+    },
+}