Fix first doc comment inside macro yield (#15050)

The following example eats the doc comment:

```cr
macro foo
  {{yield}}
end

foo do
  # doc comment
  def test
  end
end
```

This is because the first line of comment is generated on the same line as the `begin` which is inserted when using `{{yield}}`, like so:

```cr
begin # doc comment
  def test
  end
end
```

Using a newline instead of whitespace after `begin` in macro yield fixes this.

---------

Co-authored-by: Johannes Müller <straightshoota@gmail.com>
Co-authored-by: Julien Portalier <julien@portalier.com>

spec/compiler/semantic/macro_spec.cr

@@ -613,6 +613,21 @@ describe "Semantic: macro" do
       CRYSTAL
   end
 
+  it "begins with {{ yield }} (#15050)" do
+    result = top_level_semantic <<-CRYSTAL, wants_doc: true
+      macro foo
+        {{yield}}
+      end
+
+      foo do
+        # doc comment
+        def test
+        end
+      end
+      CRYSTAL
+    result.program.defs.try(&.["test"][0].def.doc).should eq "doc comment"
+  end
+
   it "can return class type in macro def" do
     assert_type(<<-CRYSTAL) { types["Int32"].metaclass }
       class Foo

src/compiler/crystal/macros/interpreter.cr

@@ -104,7 +104,7 @@ module Crystal
         if (loc = @last.location) && loc.filename.is_a?(String) || is_yield
           macro_expansion_pragmas = @macro_expansion_pragmas ||= {} of Int32 => Array(Lexer::LocPragma)
           (macro_expansion_pragmas[@str.pos.to_i32] ||= [] of Lexer::LocPragma) << Lexer::LocPushPragma.new
-          @str << "begin " if is_yield
+          @str << "begin\n" if is_yield
           @last.to_s(@str, macro_expansion_pragmas: macro_expansion_pragmas, emit_doc: true)
           @str << " end" if is_yield
           (macro_expansion_pragmas[@str.pos.to_i32] ||= [] of Lexer::LocPragma) << Lexer::LocPopPragma.new