interp: Add stderr again for jq compat

Also add tests for print* and stderr

pkg/interp/internal.jq

@@ -9,11 +9,17 @@ def println: ., "\n" | print;
 def printerr: tostring | _stderr;
 def printerrln: ., "\n" | printerr;
 
+# jq compat
 def debug:
   ( ((["DEBUG", .] | tojson) | printerrln)
   , .
   );
 def debug(f): . as $c | f | debug | $c;
+# jq compat, output to compact json to stderr and let input thru
+def stderr:
+  ( (tojson | printerr)
+  , .
+  );
 
 def _global_var($k): _global_state[$k];
 def _global_var($k; f): _global_state(_global_state | .[$k] |= f) | .[$k];

pkg/interp/testdata/debug.fqtest

@@ -13,3 +13,32 @@ null> ^D
 stderr:
 ["DEBUG",{"a":123,"b":[]}]
 ["DEBUG",{"a":123}]
+$ fq -n 'null, {a:1}, "abc" | ., "\n" | print'
+null
+{"a":1}
+abc
+$ fq -n 'null, {a:1}, "abc" | println'
+null
+{"a":1}
+abc
+$ fq -n 'null, {a:1}, "abc" | ., "\n" | printerr'
+stderr:
+null
+{"a":1}
+abc
+$ fq -n 'null, {a:1}, "abc" | printerrln'
+stderr:
+null
+{"a":1}
+abc
+$ fq -n 'null, {a:1}, "abc" | ., "\n" | stderr'
+null
+"\n"
+{
+  "a": 1
+}
+"\n"
+"abc"
+"\n"
+stderr:
+null"\n"{"a":1}"\n""abc""\n"