Add environment variable access with $NAME syntax

Lexes $NAME as TK_ENVVAR, compiles reads as __getenv("NAME") calls
and writes ($NAME = expr) as __setenv("NAME", expr) calls. Runtime
functions wrap getenv/setenv/unsetenv with automatic tostring coercion.

llex.c

@@ -50,7 +50,7 @@ static const char *const luaX_tokens [] = {
     "//", "..", "...", "==", ">=", "<=", "~=",
     "<<", ">>", "::", "<eof>",
     "<number>", "<integer>", "<name>", "<string>",
-    "<command>", "<command_interp>"
+    "<command>", "<command_interp>", "<envvar>"
 };
 
 
@@ -108,6 +108,9 @@ static const char *txtToken (LexState *ls, int token) {
     case TK_FLT: case TK_INT:
       save(ls, '\0');
       return luaO_pushfstring(ls->L, "'%s'", luaZ_buffer(ls->buff));
+    case TK_ENVVAR:
+      save(ls, '\0');
+      return luaO_pushfstring(ls->L, "'$%s'", luaZ_buffer(ls->buff));
     default:
       return luaX_token2str(ls, token);
   }
@@ -633,6 +636,18 @@ static int llex (LexState *ls, SemInfo *seminfo) {
       case '`': { /* explicit command `ls -l` with ${} interpolation */
         return read_command(ls, seminfo);
       }
+      case '$': {  /* environment variable $NAME */
+        next(ls);
+        if (lislalpha(ls->current)) {  /* $NAME */
+          do {
+            save_and_next(ls);
+          } while (lislalnum(ls->current));
+          seminfo->ts = luaX_newstring(ls, luaZ_buffer(ls->buff),
+                                            luaZ_bufflen(ls->buff));
+          return TK_ENVVAR;
+        }
+        else return '$';  /* bare $ as single-char token */
+      }
       case '.': {  /* '.', '..', '...', or number */
         save_and_next(ls);
         if (check_next1(ls, '.')) {