Improve differentiation of multiple `col_vals_expr` steps in `get_tabular_report`

Prework

• Read and abide by the pointblank code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

Great library! Thank you for all the work on it so far.

When including multiple custom column expressions (col_vals_expr) as steps, it becomes very difficult to tell them apart in get_tabular_report.

• It would be helpful if the "columns" field was populated, i.e. via pl.Expr.meta.root_names(). This looks similar to what is available in the R version (rstudio/pointblank#570)
• It would be helpful if the "step" column included the output name of the expression, if provided. i.e.
  (pl.col("a") % 1==0).alias("a_mod_1") would display as col_vals_expr(a_mod_1) instead of just col_vals_expr(). For polars, this can be done via pl.Expr.meta.output_name().
• In addition, since every step has a "brief" input, it would be nice to have an option in Validate.get_tabular_report to add a column that displays these inputs across all steps. This would also help to relabel steps that use col_vals_regex once the regular expressions become too complicated to be intuitive to consumers of the report.