Fix --with-shell not handling quoted arguments correctly

Fix #4709

Use go-shellwords instead of strings.Fields to parse --with-shell,
so paths with spaces can be properly quoted.

  ln -s /bin/bash "/tmp/ba sh"

  fzf --with-shell='/tmp/ba\ sh -c' --preview 'echo hello world'

  fzf --with-shell='"/tmp/ba sh" -c' --preview 'echo hello world'

CHANGELOG.md

@@ -20,6 +20,7 @@ CHANGELOG
     - Fixed AWK tokenizer not treating a new line character as whitespace
     - Fixed `--{accept,with}-nth` removing trailing whitespaces with a non-default `--delimiter`
     - Fixed OSC8 hyperlinks being mangled when the URL contains unicode characters (#4707)
+    - Fixed `--with-shell` not handling quoted arguments correctly (#4709)
 
 0.70.0
 ------