Improvement: Add a `debug` combinator

# Why?

While debugging combinatorial parsers, people often wrap a parser into debug like so:

```
def laLexer : Parsec Char String Unit String :=
s.lookAhead ((s.stringP "|" <* (void c.eol <|> c.eof)) <|> s.stringP " ")
```

becomes

```
def laLexer : Parsec Char String Unit String :=
s.lookAhead ((s.stringP "|" <* (void (c.eol >>= fun x => do
dbg_trace! x
pure x)
<|> c.eof)) <|> s.stringP " ")
```

It's error-prone. We'd rather have this monadic bind with a trace in a separate combinator!

# How?

- [ ] Write such combinator in `Common.lean`.

Start by reading Common.lean and inspect the existing parser combinators and tracing usage. Add the requested debug combinator there so parser values can be traced without an inline monadic bind, then verify it against the project’s existing checks.

Written by the indexing model from the issue text.