Don't force multiline interpolation into a single line (#1280)

Also, fixes an issue where forced single-line formatting would break for if/else
and let expressions

---------

Co-authored-by: Islon Scherer <islonscherer@gmail.com>

pkl-formatter/src/test/files/FormatterSnippetTests/input/string-interpolation.pkl

@@ -13,7 +13,7 @@ prop3 =
   \(new {
     // some comment
     foo = 1
-    
+
     // some comment
     bar = 2
   })
@@ -49,3 +49,28 @@ prop7 = "\(
 )"
 
 prop8 = "\(new { foo = 1 bar = 2 baz = 3 })"
+
+prop9 = "\(if (true) 1 else 2)"
+
+prop10 = "\(
+  if (true) 1
+  else 2
+)"
+
+prop11 = "\(
+  new {
+    1;
+    2;
+    3;
+  }
+)"
+
+// single line expressions are not broken up
+prop12 = "Some \(if (true) 1 else 2) reeeaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaallly long string with interpolation"
+
+// multi-line expressions are preserved
+prop13 = "Some \(
+  if (true)
+    1
+  else 2
+  ) reeeaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaallly long string with interpolation"

pkl-formatter/src/test/files/FormatterSnippetTests/output/multi-line-strings.pkl

@@ -1,6 +1,8 @@
 foo =
   """
-  asd \(new { bar = 1 }) asd
+  asd \(new {
+    bar = 1
+  }) asd
   """
 
 bar =

pkl-formatter/src/test/files/FormatterSnippetTests/output/string-interpolation.pkl

@@ -50,3 +50,29 @@ prop7 =
   )"
 
 prop8 = "\(new { foo = 1; bar = 2; baz = 3 })"
+
+prop9 = "\(if (true) 1 else 2)"
+
+prop10 =
+  "\(if (true)
+    1
+  else
+    2)"
+
+prop11 =
+  "\(new {
+    1
+    2
+    3
+  })"
+
+// single line expressions are not broken up
+prop12 =
+  "Some \(if (true) 1 else 2) reeeaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaallly long string with interpolation"
+
+// multi-line expressions are preserved
+prop13 =
+  "Some \(if (true)
+    1
+  else
+    2) reeeaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaallly long string with interpolation"