indents(go): improve @branch rules

- Don't branch at `case`
- Let `import_spec_list`/`var_declaration` behave like
  `const_declaration`

Fixes #2166

tests/indent/go/multiple-vars.go

@@ -0,0 +1,19 @@
+package main
+
+var (
+	thing    = 1
+	thingTwo = 2
+) // <-- This paren should be at 0 instead of indented
+
+var (
+	thing    = 1
+	thingTwo = 2
+)
+
+func main() {
+	// It should be
+	var (
+		thing    = 1
+		thingTwo = 2
+	)
+}