* core-load-paths.el: New spacemacs-state-directory to store user state files

[sunlin7]

To fix #7818, a new variable spacemacs-state-directory is introduced.

NOTE:
It's part of work on XDG_* directories supports. #3589.
It's pre-work for #16706.

[fnussbaum]

This PR may be a candidate where it would make sense to postpone the merge until after we release a new stable version, see #16775.

My current plan (open to suggestions) is to use a directory scheme that is compatible with the no-littering package (and with XDG in a broad sense, though no-littering does not use a cache directory), and add a user option (disabled by default) to use no-littering. By default, the old directories should probably be used, as users will likely have to manually/semi-automatically migrate their files to the new directories.

The following table contains most instances of files that are currently put into the cache directory. The second column specifies where they would end up with the current state of this PR (subject to change), and the third column is where no-littering would put them. The no-littering package uses two customizable locations: etc, and var. The etc directory could be something like dotspacemacs-directory/etc/, and var could be spacemacs-state-directory, for example.

	PR	no-littering
spacemacs-version--last-startup-check-file	state	-
spacemacs-revision--file	state	-
spacemacs--last-emacs-version-file	state	-
bm-repository-file	state	var
tramp-persistency-file-name	state	var
bookmark-default-file	state	var
recentf-save-file	state	var
savehist-file	state	var
save-place-file	state	var
desktop-dirname	state	var
added to list: desktop-path	state	var (*)
projectile-known-projects-file	state	var
persistent-scratch-save-file	state	var
configuration-layer--last-dotspacemacs-configuration-layers-file	cache	-
configuration-layer-stable-elpa-directory	cache	-
configuration-layer--rollback-root-directory	cache	-
quelpa-dir	cache	-
spacemacs-buffer--cache-file	cache	-
spacemacs-auto-save-directory	cache	-
spacemacs-dump-directory	cache	-
spacemacs--custom-file	cache	-
transient-history-file	cache	var
transient-levels-file	cache	etc
forge-database-file	cache	var
srecode-map-save-file	cache	var
semanticdb-default-save-directory	cache	var
pyim-directory	cache	-
youdao-dictionary-search-history-file	cache	-
spacemacs-games-cache-directory	cache	-
emojify-emojis-dir	cache	var
xkcd-cache-dir	cache	var
smex-save-file	cache	var
org-clock-persist-file	cache	var
org-id-locations-file	cache	var
org-publish-timestamp-directory	cache	var
lsp-server-install-dir	cache	var
image-dired-dir	cache	var
twitter-images-directory	cache	-
pandoc-data-dir	cache	etc
eshell-directory-name	cache	var
eshell-z-freq-dir-hash-table-file-name	cache	-
ac-comphist-file	cache	var
company-statistics-file	cache	var
erc-log-channels-directory	cache	var
erc-image-images-path	cache	-
erc-tweet-cache-dir	cache	-
erc-yt-cache-dir	cache	-
rcirc-log-directory	cache	var
spacemacs-layouts-directory	cache	-
projectile-cache-file	cache	var
ido-save-directory-list-file	cache	var
request-storage-directory	cache	var
undo-tree-history-directory-alist	cache	(var)
undo-fu-session-directory	cache	var
open-junk-file-format	cache	-
abbrev-file-name	cache	etc
add to list: recentf-exclude	cache	-
url-configuration-directory	cache	var
evil-tutor-working-directory	cache	-
anaconda-mode-installation-directory	cache	var
meghanada-server-install-dir	cache	var
cider-repl-history-file	cache	var
rake-cache-file	cache	-
geben-temporary-file-directory	cache	var
ietf-docs-cache	cache	-
parinfer-rust-library-directory	cache	-

There are some other files (not in the table) which are currently put into dotspacemacs-directory and spacemacs-private-directory by default, those should also be audited.