feat: Define debug/1 in terms of debug/0

ie. so it can be used for logging debug messages.

This modifies `debug` to `tee` the `stdin` to `stdout` without consuming it,
while printing the debug messages to `stderr` if `LOG_LEVEL=DEBUG` is set in `$ENV`.

Example usage:
```
seq 2 | LOG_LEVEL=DEBUG jq '
  debug({$ENV})                    |
  debug({line: input_line_number}) |
  debug("here1: .=\(.)")           |
  . * 10                           |
  debug("here2: .=\(.)")           |
  . + 1
'
```
=>
```
["DEBUG:",{"ENV":{"PWD":"/tmp","SHELL":"/bin/zsh","PATH":"/usr/bin:/bin", ...}}]
["DEBUG:",{"line":1}]
["DEBUG:","here1: .=1"]
["DEBUG:","here2: .=10"]
11
["DEBUG:",{"ENV":{"PWD":"/tmp","SHELL":"/bin/zsh","PATH":"/usr/bin:/bin", ...}}]
["DEBUG:",{"line":2}]
["DEBUG:","here1: .=2"]
["DEBUG:","here2: .=20"]
21
```

Or without `LOG_LEVEL=DEBUG`:
```
seq 2 | jq '
  debug({$ENV})                    |
  debug({line: input_line_number}) |
  debug("here1: .=\(.)")           |
  . * 10                           |
  debug("here2: .=\(.)")           |
  . + 1
'
```
=>
```
11
21
```

src/builtin.jq

@@ -1,5 +1,6 @@
 def halt_error: halt_error(5);
 def error(msg): msg|error;
+def debug(msg): . as $dot | if env.LOG_LEVEL == "DEBUG" then msg | debug | $dot else . end;
 def map(f): [.[] | f];
 def select(f): if f then . else empty end;
 def sort_by(f): _sort_by_impl(map([f]));