Instead of requiring a flag to enable newline trimming, invert it so the
flag (now `--no-trim-newlines`) disables newline trimming. This way our
default behavior matches that of sh's `"$(cmd)"`.
Also change newline trimming to trim all newlines instead of just one,
again to match sh's behavior.
The `string collect` subcommand behaves quite similarly in practice to
`string split0 -m 0` in that it doesn't split its output, but it also
takes an optional `--trim-newline` flag to trim a single trailing
newline off of the output.
See issue #159.
fish_indent_lexer formats lines not starting with a prompt indicator
as output, as long as there is a prompt indicator elsewhere.
So these tags are useless and wrong.
See #5696.
[ci skip]
This both formats it as a code-block, and adds the synopsis of each
subcommand to the corresponding section again so you don't need to
scroll back-and-forth so much.
[ci skip]
