Add project switching functionality

Add project switching functionality

* lisp/progmodes/project.el: Require subr-x.
(project--transient-p, project--ensure-file-exists)
(project--read-project-list, project--ensure-read-project-list)
(project--write-project-list)
(project--add-to-project-list-front)
(project--remove-from-project-list, project-find-project)
(project-switch-project-find-file, project-switch-project-dired)
(project-switch-project-eshell, project-add-switch-command)
(project--keymap-prompt, project-switch-project): New functions.
(project--list, project-switch-keymap): New variables.
(project-current): Call 'project-find-project' when no project is
current.