Allow all buffer names to be customized

[treydavis]

A single customizable function for all buffer names

[ericdallo]

A couple of issues:

• There are calls like here in eca.el that pass session positionally but eca-generate-buffer-name-default-function declares it as &key session — throws Keyword argument ... not one of (:session :chat-id) at runtime. Breaks eca--log-error, eca-show-errors, eca-stop (via eca--emacs-errors-exit) and eca-debug-nrepl-connect.
• eca-chat--closed is private to eca-chat.el but now also setq-local'd from eca.el and eca-process.el. byte-compiler also warns reference to free variable 'eca-generate-buffer-name-function' in eca-chat/eca-process/eca-settings due to load order. Could we move both the flag and the defcustom to eca-util.el?

[treydavis]

Thanks for the review.

• Now calling eca-generate-buffer-name-default-function consistently with keyword args
• moved eca-generate-buffer-name-function, eca-generate-buffer-name-default-function and eca-chat-closed into eca-util.el. (renamed eca-chat--closed to eca-chat-closed)

[ericdallo]

Couple of things before this merges though:

• Master moved since you opened this - Hitting enter (bound to eca-chat--key-pressed-return) has no effect #242 (eca-doctor) added four uses of eca-chat--closed in eca-chat.el (around 4277/4317/4347/4383). After the rename they'll silently read an unbound var, so this needs a rebase + sweep of those call sites.
• The three :closed cleanup loops in eca-chat.el / eca-process.el / eca.el used to each be scoped to one buffer family via a name regexp. Switching them all to (buffer-local-value 'eca-chat-closed b) means whichever loop runs first will also kill the other families' :closed buffers. Probably wants to be scoped per family, or the flag split per family.
• Since eca-chat-closed is now also set on the stderr and emacs-errors buffers, the name and its docstring don't really fit anymore - maybe just eca-buffer-closed?

Nits: settings buffer-name field order silently flipped from <...:tab-key:session-id> to <...:session-id:tab-key> because tab-key is riding on :chat-id - either restore the order or add a dedicated :tab-key / :suffix keyword. Also missing a CHANGELOG.md entry for the new defcustom and the variable rename.