add helpgen entrypoint

src/build_config.zig

@@ -135,6 +135,7 @@ pub const Artifact = enum {
 /// Therefore, main.zig uses this to switch between the different entrypoints.
 pub const ExeEntrypoint = enum {
     ghostty,
+    helpgen,
     mdgen_ghostty_1,
     mdgen_ghostty_5,
 };

src/main.zig

@@ -37,6 +37,7 @@ pub fn main() !MainReturn {
     if (comptime build_config.artifact == .exe) entrypoint: {
         switch (comptime build_config.exe_entrypoint) {
             .ghostty => break :entrypoint, // This function
+            .helpgen => try @import("helpgen.zig").main(),
             .mdgen_ghostty_1 => try @import("build/mdgen/main_ghostty_1.zig").main(),
             .mdgen_ghostty_5 => try @import("build/mdgen/main_ghostty_5.zig").main(),
         }