chore: align isInternalName with upstream version (leanprover-commu…

…nity#796)

* chore: align isInternalName with upstream version

* doc-string

Batteries/Lean/Name.lean

@@ -16,8 +16,13 @@ Generally, user code should not explicitly use internal names.
 def isInternalDetail : Name → Bool
   | .str p s     =>
     s.startsWith "_"
-      || s.startsWith "match_"
-      || s.startsWith "proof_"
+      || matchPrefix s "eq_"
+      || matchPrefix s "match_"
+      || matchPrefix s "proof_"
       || p.isInternalOrNum
   | .num _ _     => true
   | p            => p.isInternalOrNum
+where
+  /-- Check that a string begins with the given prefix, and then is only digit characters. -/
+  matchPrefix (s : String) (pre : String) :=
+    s.startsWith pre && (s |>.drop pre.length |>.all Char.isDigit)