Silently accept forward slash for special commands

[rolandwalker]

Description

Silently accept forward slash for special commands.

• commands such as \? can be entered as /?
• commands heretofore without a backslash such as help can be entered as /help

Thus all special commands can be entered, uniformly, with an introductory forward slash, yet \dt can remain available, similar to Postgres, and plain exit can remain, similar to the vendor MySQL client.

This helps both new learners and those with muscle memory.

The only limitation to the introductory forward slash is that MyCli could never use /* (comment-start) as a special command.

For trailing commands, there is also a limitation. Trailing commands such as \edit cannot also be entered as /edit, since a string can be an expression, which is valid SQL:

select 1 as edit group by 2/edit;

SpecialCommand already had a "hidden" property, which is used here to keep the forward-slash forms from being advertised in help, so they can be considered to be experimental. However, the plan would be to advertise the forward-slash forms quite soon in MyCli 2.x. Hidden forward-slash commands are also still completable.

Some small incidental improvements are made to the detection of special commands by case, and on the acceptance of /? <term> for help on a term.

Motivation: the forward slash has become something of an industry standard for out-of-band commands, consistent across modern applications such as Slack, Codex, or Claude. It also has a long heritage in, for example, IRC. On most keyboards, it is easier to type than the backslash.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .