Limit the number of items in the jumplist (#4750)

Previously, jumplists could grow unchecked. Every transaction is
applied to jumplist selections to ensure that they are up to date
and within document bounds, so this would cause every edit to become
more expensive as jumplist lengths increased throughout a session.
Setting a maximum number of entries limits the cost.

Vim and Neovim limit their jumplists:

* https://github.com/vim/vim/blob/b298fe6cbae3b240b10dbd55d9c38d0cc8e033d3/src/structs.h#L141
* https://github.com/neovim/neovim/blob/e8cc489accc435076afb4fdf89778b64f0a48473/src/nvim/mark_defs.h#L57

Notably, Kakoune does not. In Kakoune, changes are applied to jumplist
entries lazily as you hit `<C-o>`/`<C-i>` though, so Kakoune doesn't
have the same growing cost concerns. Kakoune also does not have a
concept of a View which limits the cost further.

Vim and Neovim limit to 100. This seems unreasonably high to me so I've
set this to 30 to start. We can increase if this is problematically
low.

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