Add --markdown-tables to lay out Markdown tables - #4008

Closes #4007.

Adds --markdown-tables, which lays out the tables in a Markdown file so the columns line up, and draws a border around them.

                                  ┌───────────┬─────────────┬───────┬───────┐
| Component | Status | Owner |    │ Component │   Status    │ Owner │ Notes │
|---|:---:|---|---:|         ->   ╞═══════════╪═════════════╪═══════╪═══════╡
| parser | done | alice | 12 |    │ parser    │    done     │ alice │    12 │
| renderer | in progress | b |    │ renderer  │ in progress │ bob   │     3 │
                                  └───────────┴─────────────┴───────┴───────┘

Cells keep the alignment the delimiter row asks for, and cell widths are measured with unicode-width, so wide characters line up too. The border shape is the one comfy-table and rich settled on.

How it works

A table cannot be laid out until all of its rows have been seen, so TableRenderer in the new src/markdown_table.rs holds the lines back until the table ends, then rewrites them. The rewrite happens before highlighting, so syntect and the decorations carry on as normal.

The two border lines are the only lines that do not exist in the file. They are printed the way a wrapped line is, as a continuation with an empty gutter, so every real line keeps its own number.

   1 Before the table.
   2
     ┌─────────┬───────┬───────┐
   3 │ Fruit   │ Price │ Stock │
   4 ╞═════════╪═══════╪═══════╡
   5 │ Apples  │  1.20 │  yes  │
     └─────────┴───────┴───────┘

Notes

It is off by default, because it changes the text that gets printed and that would work against the drop-in cat goal. Without the flag the output is byte for byte the input, and the Markdown syntax test fixture is unchanged.

It has no effect under --unbuffered, since holding lines back is the opposite of what that mode is for, and no effect inside fenced or indented code blocks.

PrettyPrinter::markdown_tables exposes the same thing to library users.

Known limits

Tabs inside cells are measured before expand_tabs runs, so a cell containing a tab can still be off. A table wider than the terminal wraps like any other long line. With --line-range, only the visible rows are measured, so the columns can come out narrower than in a full render.