tempel: Theme tempel-path

Placing the s-expressions lisp data file below "etc/" since it's not
an auto-generated file.

Co-authored-by: Jonas Bernoulli <jonas@bernoul.li>
  Use a sub-directory, which would avoid breakage if we later decide
  that we should use wildcards ("tempel/*/*.eld") instead of a single
  file.

no-littering.el

@@ -336,6 +336,7 @@ This variable has to be set before `no-littering' is loaded.")
     (setq svg-lib-icons-dir                (var "svg-lib/icons/"))
     (setq sx-cache-directory               (var "sx/cache/"))
     (setq tabspaces-session-file           (var "tabspaces-session.eld"))
+    (setq tempel-path                      (etc "tempel/templates.eld"))
     (setq tldr-directory-path              (var "tldr/"))
     (setq transient-history-file           (var "transient/history.el"))
     (setq transient-levels-file            (etc "transient/levels.el"))