Fix indentation of `let` (#868) (#2001) (#2348)

Signed-off-by: Wilson Snyder <wsnyder@wsnyder.org>

verible/verilog/formatting/formatter_test.cc

diff --git a/verible/verilog/formatting/formatter_test.cc b/verible/verilog/formatting/formatter_test.cc
index 5205a01..55c9509 100644
--- a/verible/verilog/formatting/formatter_test.cc
+++ b/verible/verilog/formatting/formatter_test.cc
@@ -4492,6 +4492,18 @@
     {"typedef my_class #(.P(P)) my_class_t;\n",
      "typedef my_class #(.P(P)) my_class_t;\n"},
 
+    // let declarations each stay on their own line
+    {"module t;\n"
+     "let OFF = 4;\n"
+     "let UNIQUE = 32;\n"
+     "let PP(a) = 30 + a;\n"
+     "endmodule\n",
+     "module t;\n"
+     "  let OFF = 4;\n"
+     "  let UNIQUE = 32;\n"
+     "  let PP(a) = 30 + a;\n"
+     "endmodule\n"},
+
     // package test cases
     {"package fedex;localparam  int  www=3 ;endpackage   :  fedex\n",
      "package fedex;\n"

verible/verilog/formatting/tree-unwrapper.cc

diff --git a/verible/verilog/formatting/tree-unwrapper.cc b/verible/verilog/formatting/tree-unwrapper.cc
index a6601ac..bc1af07 100644
--- a/verible/verilog/formatting/tree-unwrapper.cc
+++ b/verible/verilog/formatting/tree-unwrapper.cc
@@ -801,6 +801,7 @@
     case NodeEnum::kPreprocessorUndef:
     case NodeEnum::kTFPortDeclaration:
     case NodeEnum::kTypeDeclaration:
+    case NodeEnum::kLetDeclaration:
     case NodeEnum::kNetTypeDeclaration:
     case NodeEnum::kForwardDeclaration:
     case NodeEnum::kInterfaceClassMethod: