recognize \bgroup as an opener for readBalanced

[dginev]

Fixes #2335

Based on a small pdftex test, the current latexml behavior is correct.
We are just emitting an extra error for the case where \bgroup was used as the opener for a balanced read with $require_open set to true.

\def\foo#1{bar #1 baz}

% does NOT treat \bgroup as {
\foo\bgroup hello}

% treats \bgroup as {
T5.8:
\expanded\bgroup\meaning\egroup}

% also treats \bgroup as {
\lowercase\bgroup SMALL}

\bye

Recognizing tokens that were \let to { seems to be appropriate, even if I am not sure it matches 100% the original implementation.

I also updated the POD with slightly more recent information.

[dginev]

Background: I freshly reproduced this error in the Rust port, so decided to come back and understand the root cause better.