Instead of starting the default shell and sending it text (hoping
it will support POSIX like shell syntax), just use the `env` utility
to set the correct environment and exec kakoune.
this uses 'export' to set $TMPDIR instead; fish provides an 'export'
function for bash-compatibility, so this should work for both shells.
note that prior to this change, opening new panes with fish did not
work, as fish does not support `FOO=1 bar` syntax.
Level out the builtin commands loaded at startup in terms of format and
expressiveness. The following convention was followed:
* commands that take more than one argument have to be described along
with their parameters prior to the actual documentation, otherwise the
docstring consists in a capitalized sentence
e.g. `command <arg1>: do something`
* optional arguments are enclosed in square brackets, to comply with the
format used for hardcoded commands
e.g. `cd [<directory>]`
* describe the effects of the command in the documentation string and
omit implementation details unless they are relevant. Usually command
names include the name of the tool they use, so they don't need to be
redundantly mentioned
e.g. `tmux-new-pane <arguments>: open a new pane`
* document the format the parameters to the commands, or list them if
they are to be chosen among a list of static values (c.f. `spell.kak`)
