builtin: Add zig_version

This will enable code to perform version checks and make it easier to
support multiple versions of Zig.

Within the SemVer implementation, an intermediate value needed to be
coerced to a slice to workaround a comptime bug.

Closes #6466

src/Compilation.zig

@@ -2703,6 +2703,11 @@ pub fn generateBuiltinZigSource(comp: *Compilation, allocator: *Allocator) ![]u8
         \\pub const arch = Target.current.cpu.arch;
         \\/// Deprecated
         \\pub const endian = Target.current.cpu.arch.endian();
+        \\
+        \\/// Zig version. When writing code that supports multiple versions of Zig, prefer
+        \\/// feature detection (i.e. with `@hasDecl` or `@hasField`) over version checks.
+        \\pub const zig_version = try @import("std").SemanticVersion.parse("{s}");
+        \\
         \\pub const output_mode = OutputMode.{};
         \\pub const link_mode = LinkMode.{};
         \\pub const is_test = {};
@@ -2714,6 +2719,7 @@ pub fn generateBuiltinZigSource(comp: *Compilation, allocator: *Allocator) ![]u8
         \\    .features = Target.{}.featureSet(&[_]Target.{}.Feature{{
         \\
     , .{
+        build_options.version,
         std.zig.fmtId(@tagName(comp.bin_file.options.output_mode)),
         std.zig.fmtId(@tagName(comp.bin_file.options.link_mode)),
         comp.bin_file.options.is_test,