firenvim.vim: attempt to use nvim in $PATH if found

According to https://repology.org/project/neovim/versions , most
supported distributions now ship a version of Neovim recent enough to
run Firenvim, so hopefully even outdated versions from distributions
should not cause us problems.

This change is still slightly risky because according to repology still,
Ubuntu 20.04 still ships neovim 0.4.3, which Firenvim might not work
with. According to https://ubuntu.com/about/release-cycle , Ubuntu 20.04
will stop being supported in April 2025, so let's take a gamble and
attempt to solve issues such as #1614.

autoload/firenvim.vim

@@ -692,9 +692,13 @@ function! s:get_executable_content(data_dir, prolog) abort
                                 \ s:capture_env_var('XDG_CACHE_HOME') .
                                 \ s:capture_env_var('XDG_RUNTIME_DIR') .
                                 \ s:capture_env_var('NVIM_APPNAME') .
+                                \ "if command -v nvim >/dev/null 2>/dev/null; then\n" .
+                                \ "  FIRENVIM_NVIM_BINARY=nvim\n" .
+                                \ "else\n" .
+                                \ '  FIRENVIM_NVIM_BINARY=' . s:get_progpath() . "\n" .
+                                \ "fi\n" .
                                 \ a:prolog . "\n" .
-                                \ "exec '" . s:get_progpath() .
-                                  \ "' --headless " . l:stdioopen .
+                                \ 'exec "$FIRENVIM_NVIM_BINARY" --headless ' . l:stdioopen .
                                   \ " --cmd 'let g:started_by_firenvim = v:true' " .
                                   \ "-c 'try|" .
                                       \ 'call firenvim#run()|' .