floats: add NormalFloat highlight and 'nonumber' default

src/nvim/highlight_defs.h

@@ -90,6 +90,7 @@ typedef enum {
   , HLF_0           // Whitespace
   , HLF_INACTIVE    // NormalNC: Normal text in non-current windows
   , HLF_MSGSEP      // message separator line
+  , HLF_NFLOAT      // Floating window
   , HLF_COUNT       // MUST be the last one
 } hlf_T;
 
@@ -142,6 +143,7 @@ EXTERN const char *hlf_names[] INIT(= {
   [HLF_0] = "Whitespace",
   [HLF_INACTIVE] = "NormalNC",
   [HLF_MSGSEP] = "MsgSeparator",
+  [HLF_NFLOAT] = "NormalFloat",
 });