Systematically replacing `\linebreak` with `\brk` or `\-`

[Eisenwave]

We have a 113 uses of manual \linebreak in the draft right now, and I'm not convinced that any of them are strictly necessary.

The problem with \linebreak is that unlike \brk{}, it introduces a forced line break, and if anything about the surrounding text changes, breaking at that point may become unnecessary or look totally wrong. It is better to introduce \brk{} for line break opportunities, or \- for soft hyphens. At worst, those have no effect.

Take for example:

draft/source/exec.tex

Lines 1365 to 1369 in 06614c2

	If all of the types
	\tcode{\exposid{COMPL-DOMAIN}(set_value_t)},
	\tcode{\exposid{COMPL-DOMAIN}(set_error_t)}, and\linebreak
	\tcode{\exposid{COMPL-DOMAIN}(set_stopped_t)} are ill-formed,
	\tcode{completion-domain<Default>(sndr)} is

Surely, paragraph justification could have been made happy by introducing a \- or \brk{} somewhere in those \tcodes.

One may argue that it's a waste of time/effort to replace \linebreaks, but I think it sets a bad precedent if we have any of these laying around. People copy that bad precedent. I'm willing to bet that some of these 100+ \linebreaks already look bad in the PDF.

We might even be able to ban the use of \linebreak outright via checks if all goes well.