Add m textobject to select closest surround pair

[sudormrfbin]

m now acts as a wildcard surround textobject, so for example with this text

[abc (def) ghi]

If the cursor is on c, mim will select everything inside [], and if it's on e it will select inside (), without manually requiring either mi( or mi[.

[archseer]

Looks like the test suite needs some adjustments?

[pickfire]

Rather than doing this, I think doing something more specific is better, like b for (, q for ' and similar, m is covers too much and the scope in too broad, user need to guess the context I believe to have this working the way they want.

[sudormrfbin]

I've personally never needed this with vim-sandwich (from which I borrowed the m behavior), since it is supposed to be a catch-all -- that is it's sole purpose. If the closest pair isn't what I want I use the explicit form of mi(, etc instead of mim. The only drawback could be that we currently base the nearest pair matching off of the characters from the PAIRS constant which has some non-standard pairs:

helix/helix-core/src/surround.rs

Lines 6 to 14 in 1237b1a

	pub const PAIRS: &[(char, char)] = &[
	('(', ')'),
	('[', ']'),
	('{', '}'),
	('<', '>'),
	('«', '»'),
	('「', '」'),
	('（', '）'),
	];

We could explore keys like b and q if there is interest in doing so, but I think m is a good first step.