Add config and attribute to set the number of array values per line

In programming, arrays of data are sometimes formatted with X values per line, usually a multiple of 10 or 2. This allows you to easily find and inspect the data of a specific index by counting the lines and values on the target line.

The number of values per line depends on the nature of the data, so a global config is basically a default. You also need a way to set the config of individual arrays.

Currently I have to use #[rustfmt::skip] and format it manually, but I really want it to be automatic.

Example:

const BAD: u8 = 0xff;

#[rustfmt::skip]
static base64val: [u8; 128] = [
    BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD,
    BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD,
    BAD,BAD,BAD,BAD, BAD,BAD,BAD,BAD, BAD,BAD,BAD, 62, BAD,BAD,BAD, 63,
     52, 53, 54, 55,  56, 57, 58, 59,  60, 61,BAD,BAD, BAD,BAD,BAD,BAD,
    BAD,  0,  1,  2,   3,  4,  5,  6,   7,  8,  9, 10,  11, 12, 13, 14,
     15, 16, 17, 18,  19, 20, 21, 22,  23, 24, 25,BAD, BAD,BAD,BAD,BAD,
    BAD, 26, 27, 28,  29, 30, 31, 32,  33, 34, 35, 36,  37, 38, 39, 40,
     41, 42, 43, 44,  45, 46, 47, 48,  49, 50, 51,BAD, BAD,BAD,BAD,BAD
];

If possible I would like an option to set the maximum number of array values per line and an attribute to configure individual arrays. Something like:

#[rustfmt::config(max_array_values_per_line = 16)]
static base64val: [u8; 128] = [
    BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD,
    BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD,
    BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, BAD, 62, BAD, BAD, BAD, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, BAD, BAD, BAD, BAD, BAD, BAD,
    BAD, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
    15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, BAD, BAD, BAD, BAD, BAD,
    BAD, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
     41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, BAD, BAD, BAD, BAD, BAD
];

If you feel like it, an option that aligns values "vertically" (to the left, to the right or centered) is welcome. 😉

No files, tests, or entry points are named. Start by locating rustfmt's array-formatting and configuration handling, then determine how a global limit and per-array attribute can be specified and tested; done means arrays honor both settings without requiring rustfmt::skip.