runtime(dist): Make dist/vim.vim work properly when lacking vim9script support (...

runtime(dist): Make dist/vim.vim work properly when lacking vim9script support (#13487)

`:return` cannot be used outside of `:function` (or `:def`) in older Vims
lacking Vim9script support or in Neovim, even when evaluation is being skipped
in the dead `:else` branch.

Instead, use the pattern described in `:h vim9-mix`, which uses `:finish` to end
script processing before it reaches the vim9script stuff.

Signed-off-by: Sean Dewar <seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>