Fix VimuxOpenRunner not working for versions of tmux < 3.4

[jmriego]

This is a fix for #225 to allow VimuxOpenRunner to work for versions of tmux < 3.1

The breaking change was introduced in #217 and was caused because even though the -l flag existed for a long time, it never supported percentage signs until tmux 3.1. Relevant part of their CHANGES

* Add support for percentage sizes for resize-pane ("-x 10%"). Also change
  split-window and join-pane -l to accept similar percentages and deprecate the
  -p flag.

This is an old tmux version so in my opinion the most logical thing is to support by default the newer tmux syntax. The default for g:VimuxHeight is 20%.
This won't support the old tmux flag -p so for older versions you'd only be able to specify the size of the split in columns/lines.

[mvanderkamp]

Thanks! This could cause some confusion for anyone who has set this variable but that is easily fixed by users.