add clear() function that replaces Main

a link to the previous Main called LastMain is provided.
`using LastMain.Package` works to restore access to loaded libraries.

fixes #1195, fixes #2385
return nothing from clear()
NEWS and documentation for `clear`
rename `clear` to `workspace`, which seems apt since it makes a new
workspace, and I find it hard to explain the function without using
that word.
Merge pull request #7487 from JuliaLang/jb/clear

add workspace() function that replaces Main