feat(commands): ensure_selections_forward

[matoous]

Add ensure_selections_forward command and bind it to A-: by default.

Fixes: #1332

[dead10ck]

Nice work. Should we also update the flip_selections command to make use of Range::flip for consistency?

[matoous]

@dead10ck thanks for suggestion, done.

[dead10ck]

Looks good to me!

[archseer]

Range is Copy so self can be used:

Suggested change

	pub fn flip(&self) -> Self {
	pub fn flip(self) -> Self {

Doesn't seem to change the compiler output though. I also attempted to use

    pub fn flip(mut self) -> Self {
        std::mem::swap(&mut self.head, &mut self.anchor);
        self
    }

to see if it would generate better assembly but it ended up worse: https://rust.godbolt.org/z/rP8vva9G6