refactor(commands): add a substitute constructor

This commit recuces some repetitiveness from creating substitution
literals in tests.

src/bot.zig

@@ -269,9 +269,7 @@ test "execute substitution" {
     bot.hear(msg);
 
     // execute substitution
-    const cmd = UserCommand{
-        .substitute = .{ .author = "jassob", .needle = "What", .replacement = "what" },
-    };
+    const cmd = UserCommand.init_substitute("jassob", "What", "what", false);
     const response = try bot.execute(&cmd, null, "#test");
 
     // expect response matching the correct message
@@ -292,9 +290,7 @@ test "execute substitution with no matching needle" {
     bot.hear(msg);
 
     // execute substitution
-    const cmd = UserCommand{
-        .substitute = .{ .author = "jassob", .needle = "something else", .replacement = "weird" },
-    };
+    const cmd = UserCommand.init_substitute("jassob", "something else", "weird", false);
     try std.testing.expectError(Error.NoMessage, bot.execute(
         &cmd,
         null,

src/commands.zig

@@ -9,6 +9,15 @@ pub const UserCommand = union(enum) {
     /// !help
     help: void,
 
+    pub fn init_substitute(author: []const u8, needle: []const u8, replacement: []const u8, all: bool) UserCommand {
+        return .{ .substitute = .{
+            .author = author,
+            .needle = needle,
+            .replacement = replacement,
+            .all = all,
+        } };
+    }
+
     pub fn parse(nick: []const u8, text: []const u8) ?UserCommand {
         const original = Parser.init(text);
         if (original.consume_str("!help")) |_| {
@@ -38,14 +47,7 @@ pub const UserCommand = union(enum) {
                 return null;
             }
             parser, const correction = result.?;
-            return .{
-                .substitute = .{
-                    .author = nick,
-                    .needle = typo,
-                    .replacement = correction,
-                    .all = false,
-                },
-            };
+            return .init_substitute(nick, typo, correction, false);
         }
         return null;
     }