support ".zig_version" in build.zig.zon

I've create a proposal to officially support and require .zig_version
in build.zig.zon.  However, I'd like to start using this now.  This
should be ok for now as zig currently allows unknown fields in the
zon file.

Author: marler8997

build.zig

@@ -409,6 +409,25 @@ fn addTests(
         t.run.expectStdOutEqual("0.13.0\n");
     }
 
+    {
+        const write_files = b.addWriteFiles();
+        _ = write_files.add("build.zig", "");
+        _ = write_files.add("build.zig.zon",
+            \\.{
+            \\    .minimum_zig_version = "0.13.0",
+            \\    .zig_version = "0.14.0",
+            \\}
+            \\
+        );
+        const t = test_factory.add(.{
+            .name = "test-zig-version",
+            .input_dir = .{ .path = write_files.getDirectory() },
+            .options = .nosetup,
+            .args = &.{"version"},
+        });
+        t.run.expectStdOutEqual("0.14.0\n");
+    }
+
     {
         const write_files = b.addWriteFiles();
         _ = write_files.add("build.zig", "");

src/main.zig

@@ -150,12 +150,10 @@ fn anyzigLog(
 
 const Extent = struct { start: usize, limit: usize };
 
-fn extractMinZigVersion(zon: []const u8) ?Extent {
-    return extractZigVersion(zon, ".minimum_zig_version");
-}
-fn extractMachZigVersion(zon: []const u8) ?Extent {
-    return extractZigVersion(zon, ".mach_zig_version");
-}
+const key_minimum_zig_version = ".minimum_zig_version";
+const key_zig_version = ".zig_version";
+const key_mach_zig_version = ".mach_zig_version";
+
 fn extractZigVersion(zon: []const u8, needle: []const u8) ?Extent {
     var offset: usize = 0;
     while (true) {
@@ -249,35 +247,35 @@ fn determineSemanticVersion(scratch: Allocator, build_root: BuildRoot) !Semantic
     };
     defer scratch.free(zon);
 
-    if (extractMachZigVersion(zon)) |version_extent| {
+    for ([_][]const u8{
+        key_mach_zig_version,
+        key_zig_version,
+        key_minimum_zig_version,
+    }) |key_version| {
+        const version_extent = extractZigVersion(zon, key_version) orelse continue;
         const version = zon[version_extent.start..version_extent.limit];
-        if (!std.mem.endsWith(u8, version, "-mach")) errExit(
-            "expected the .mach_zig_version value to end with '-mach' but got '{s}'",
-            .{version},
-        );
+
+        if (key_version.ptr == key_mach_zig_version.ptr) {
+            if (!std.mem.endsWith(u8, version, "-mach")) errExit(
+                "expected the " ++ key_mach_zig_version ++ " to end with '-mach' but got '{s}'",
+                .{version},
+            );
+        }
+
         log.info(
-            "zig mach version '{s}' pulled from '{}build.zig.zon'",
-            .{ version, build_root.directory },
+            "{s} '{s}' pulled from '{}build.zig.zon'",
+            .{ key_version, version, build_root.directory },
         );
         return SemanticVersion.parse(version) orelse errExit(
-            "{}build.zig.zon has invalid .mach_zig_version \"{s}\"",
-            .{ build_root.directory, version },
+            "{}build.zig.zon has invalid {s} \"{s}\"",
+            .{ build_root.directory, key_version, version },
         );
     }
 
-    const version_extent = extractMinZigVersion(zon) orelse errExit(
+    errExit(
         "build.zig.zon is missing minimum_zig_version, either add it or run '{s} VERSION' to specify a version",
         .{@tagName(build_options.exe)},
     );
-    const minimum_zig_version = zon[version_extent.start..version_extent.limit];
-    log.info(
-        "zig version '{s}' pulled from '{}build.zig.zon'",
-        .{ minimum_zig_version, build_root.directory },
-    );
-    return SemanticVersion.parse(minimum_zig_version) orelse errExit(
-        "{}build.zig.zon has invalid .minimum_zig_version \"{s}\"",
-        .{ build_root.directory, minimum_zig_version },
-    );
 
     // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     // TODO: if we find ".{ .path = "..." }" in build.zig then we know zig must be older than 0.13.0
@@ -504,7 +502,7 @@ pub fn main() !void {
             , .{semantic_version});
             return;
         };
-        const version_extent = extractMinZigVersion(zon) orelse {
+        const version_extent = extractZigVersion(zon, key_minimum_zig_version) orelse {
             if (!std.mem.startsWith(u8, zon, ".{")) @panic("zon file did not start with '.{'");
             if (zon.len < 2 or zon[2] != '\n') @panic("zon file not start with '.{\\n");
             const f = try std.fs.cwd().createFile("build.zig.zon", .{});