New backend for math matrix printing

[waldyrious]

Currently the matrix format uses a slight variation of the default format, by using the box drawing characters and simply suppressing the top, bottom and middle lines of the table:

┌                     ┐
│ 1   false   1.0   1 │
│ 2    true   2.0   2 │
│ 3   false   3.0   3 │
└                     ┘

It would be nice if we could use actual bracket characters for the matrix backend. Unicode has dedicated characters for precisely this use case. It would look like this:

⎡ 1   false   1.0   1 ⎤
⎜ 2    true   2.0   2 ⎥
⎣ 3   false   3.0   3 ⎦

This was originally proposed in #33, but it would require more substantial changes than a mere tweak to the format, because with the extended bracket characters format the corners are not placed in a separate "row", as is the case with the current format, but rather inline with the first and last lines of actual content. Thus I'm opening a new issue to track this modification.

[thchr]

One downside to this otherwise very nice idea is that the display of ⎡ and ⎤ seems broken in e.g. VSCode's integrated terminal, cf.: The display of U+23A1 (⎡) and U+23A4 (⎤) is mangled. I guess the fallback font has these wrong, somehow; but since there's a lot of VSCode users, this may not be desirable.

EDIT: I filed an issue in VSCode repo about the character rendering issue: https://github.com/microsoft/vscode/issues/172504