libyang REFACTOR avoid constant literals

Improve readability of the code.

Includes also unification of "no break in the case" comment to "fall
through".
diff --git a/src/tree_schema_internal.h b/src/tree_schema_internal.h
index 4ee5aac..fd1a795 100644
--- a/src/tree_schema_internal.h
+++ b/src/tree_schema_internal.h
@@ -25,8 +25,15 @@
 struct lysc_ctx;
 struct lys_glob_unres;
 
+#define LY_YANG_SUFFIX ".yang"
+#define LY_YANG_SUFFIX_LEN 5
+#define LY_YIN_SUFFIX ".yin"
+#define LY_YIN_SUFFIX_LEN 4
+
 #define YIN_NS_URI "urn:ietf:params:xml:ns:yang:yin:1"
 
+#define LY_PCRE2_MSG_LIMIT 256
+
 /**
  * @brief Check module version is at least 2 (YANG 1.1) because of the keyword presence.
  * Logs error message and returns LY_EVALID in case of module in YANG version 1.0.
@@ -79,6 +86,12 @@
         LOGWRN(PARSER_CTX(CTX), "Empty argument of %s statement does not make sense.", STMT); \
     }
 
+/*
+ * Additional YANG constants
+ */
+#define Y_TAB_SPACES         8  /**< number of spaces instead of tab character */
+#define LY_TYPE_DEC64_FD_MAX 18 /**< Maximal value of decimal64's fraction-digits */
+
 /**
  * @brief List of YANG statement groups - the (sub)module's substatements
  */