Fix comment handling when looking for commands - #507

Comment lines are only skipped while evcxr looks for commands if they start at column zero, so an indented comment before a command ends the search and the command is then treated as Rust code. ctx.check(" // An indented comment\n :an_invalid_command") reports a syntax error at the colon instead of "Unrecognised command", and the same input through execute fails to compile rather than running the command.

The check in evcxr/src/code_block.rs is also too broad in the other direction. line.starts_with("//") matches /// too, so a doc comment at column zero is skipped and never reaches the compiler, and the item it documents ends up with no documentation. check_for_doc only passes because its ///this is my struct happens to be indented; put it at column zero and :doc MyStruct prints the struct with the doc text gone.

from_original_user_code now trims leading whitespace before it looks for //, and an outer doc comment no longer ends the search on its own. It is held as the point where the code starts if code follows it, and dropped if a command follows, which is what happened before. //! and //// stay plain comments: an inner doc comment would document the module evcxr generates around the cell rather than any of the user's code, and //// is not a doc comment to rustc either.

Tests:

• cargo test -p evcxr -- --test-threads 1, on macOS with rustc 1.98.0: 32 unit tests and 48 integration tests pass.
• The four new unit tests and the two new integration tests fail on main. indented_comment_before_command finds no commands, doc_comment_stays_with_its_item loses the /// line, command_after_indented_comment gets error 2:5-2:6 instead of error 2:1-2:20, and check_for_doc_starting_at_column_zero gets ctx\n\nstruct MyStruct(usize) with the doc text missing. doc_comment_before_command_is_dropped and comments_that_arent_outer_doc_comments_are_dropped pin behaviour that has not changed and pass either way.
• cargo fmt --all -- --check
• cargo clippy -p evcxr --all-targets

No new dependencies, so Cargo.lock and cackle.toml are untouched.