feat: implement format strings

include/compiler/fzl_parser.h

@@ -11,6 +11,7 @@ typedef enum
   FZL_NODETYPE_BLOCK,
   FZL_NODETYPE_CALL,
   FZL_NODETYPE_LITERAL,
+  FZL_NODETYPE_FMT_STRING,
   FZL_NODETYPE_IDENTIFIER,
   FZL_NODETYPE_CONDITIONAL,
   FZL_NODETYPE_FUNCTION,

include/compiler/fzl_token.h

@@ -40,6 +40,9 @@ typedef enum
   FZL_TOKTYPE_BOOL_LITERAL, // true
   FZL_TOKTYPE_NULL_LITERAL, // null
 
+  FZL_TOKTYPE_STR_FORMAT_START, // `$` before `"`
+  FZL_TOKTYPE_STR_FORMAT_END,   // end of string formatting sequence
+
   FZL_TOKTYPE_DAMPERSAND, // &&
   FZL_TOKTYPE_DVBAR,      // ||