Make erc-cmd-AMSG session local; add /GMSG, /AME and /GME

Make erc-cmd-AMSG session local; add /GMSG, /AME and /GME

* etc/ERC-NEWS: Mention new slash commands.
* lisp/erc/erc.el (erc-cmd-AMSG): Make it consistent with the doc
string by only affecting the current connection.
(erc-cmd-GMSG, erc-cmd-AME, erc-cmd-GME): New IRC slash commands.
* test/lisp/erc/erc-scenarios-misc-commands.el
(erc-scenarios-misc-commands--AMSG-GMSG-AME-GME): New test.
* test/lisp/erc/resources/commands/amsg-barnet.eld: New file.
* test/lisp/erc/resources/commands/amsg-foonet.eld: New file.
(Bug#68401)