Add support for unit if/else blocks

Closes #144.

corpus/unit.spicy

@@ -166,3 +166,15 @@ type Test = unit {
     on b %error {}
 };
 on Test::b(msg: string) %error {}
+
+# Unit if/else blocks.
+type X = unit {
+    if (True) {
+             a1: bytes &size=2;
+    } &max-size=42 &requires=True;
+
+    if (True) {
+             a1: bytes &size=2;
+         a1: bytes &size=2;
+    } else {} &max-size=42 &requires=True;
+};

corpus/unit.spicy.expected

@@ -191,3 +191,16 @@ type Test = unit {
     on b %error {}
 };
 on Test::b(msg: string) %error {}
+
+# Unit if/else blocks.
+type X = unit {
+    if (True) {
+        a1: bytes &size=2;
+    } &max-size=42 &requires=True;
+
+    if (True) {
+            a1: bytes &size=2;
+            a1: bytes &size=2;
+        } else {
+    } &max-size=42 &requires=True;
+};

src/query.scm

@@ -261,6 +261,7 @@
   (comment)
   (type_decl)
   (field_decl)
+  (unit_if_else)
   (unit_switch)
   (struct_decl)
   (sink_decl)
@@ -371,6 +372,25 @@
   ) @delete
 )
 
+; Unit `if`/`else` blocks.
+(unit_if_else
+  "if" @append_space
+  "{" @prepend_space
+  ("else" @prepend_space @append_space)?
+)
+(unit_if_else
+  "{" @append_indent_start @append_hardline
+  .
+  (field_decl)* ; List explicitly so we match the correct {} pair.
+  .
+  "}" @prepend_indent_end @prepend_hardline
+)
+(unit_if_else
+  (field_decl) @append_empty_softline
+  .
+  (comment)? @do_nothing
+)
+
 ; Unit `switch` statement.
 (unit_switch (expression) ")" @append_space)