imp: auto-declare builtin tag names like type: and t: (#2119)

Now using type: in account declarations or generating t: with timedot
letters won't cause the `tags` check to fail.

If a user declares any of these explicitly with a tag directive,
it does not cause an error.

hledger-lib/Hledger/Data/JournalChecks.hs

@@ -186,7 +186,7 @@ journalCheckTags j = do
                     -- where
                     --   col  = T.length (showTransactionLineFirstPart txn') + 2
                     --   col2 = col + T.length tagname - 1
-    declaredtags = journalTagsDeclared j
+    declaredtags = journalTagsDeclared j ++ builtinTags
     msg = (unlines [
       "%s:%d:"
       ,"%s"
@@ -197,6 +197,19 @@ journalCheckTags j = do
       ,"tag %s"
       ])
 
+-- | Tag names which have special significance to hledger.
+builtinTags = [
+   "type"  -- declares an account's type
+  ,"t"     -- generated by timedot letters notation
+  -- optionally generated on periodic transactions and auto postings
+  ,"generated-transaction"
+  ,"generated-posting"
+  -- used internally, not shown (but queryable)
+  ,"_generated-transaction"
+  ,"_generated-posting"
+  ,"_conversion-matched"
+  ]
+
 -- | In each tranaction, check that any conversion postings occur in adjacent pairs.
 journalCheckPairedConversionPostings :: Journal -> Either String ()
 journalCheckPairedConversionPostings j =