This follows the convention used in Emacs's own test suite and works around
annoying circular `require`s when `ert-tests` happens to be in `load-path`.
Also adjust the Makefile not to get confused by generated files.

* Makefile (SRCS): Filter out ELPA-generated files.

Instead of treating `lisp-indent-offset` specially in
`editorconfig--should-set` (to obey `editorconfig-lisp-use-default-indent`),
use a new function `editorconfig-set-indentation-lisp-mode`
for Lisp modes.

* lisp/editorconfig.el (editorconfig--should-set): Remove `size` argument.
(editorconfig-set-indentation): Adjust calls accordingly.
(editorconfig-set-indentation-lisp-mode): New function.
(editorconfig-indentation-alist): Use it.

Mostly fix compiler warnings and prefer #' to quote function names.

* editorconfig.el: Remove dependency on `nadvice` since we require
Emacs-26 which includes `nadvice` already.
Remove redundant `require`s.  Remove redundant `:group` arguments.
Move `defvar`s outside of `eval-when-compile`.
Remove "post-end trailer".
* .dir-locals.el: New file, to replace the "post-end trailer".

* ert-tests/editorconfig-core-handle-tests.el (editorconfig--fixtures):
Rename from `fixtures`.  Change all uses.

* ert-tests/editorconfig-tests.el (test-editorconfig)
(test-local-variables, test-hack-properties-functions):
Use `bound-and-true-p` when it's not guaranteed that the var will
be defined.

Don't re-set variables just because `read-only-mode` is (de)activated,
and with Emacs-30's hooks it would be even less natural to do.
Also `buffer-read-only` can change without calling `read-only-mode`,
so better test it dynamically when we try to trim whitespace.

* lisp/editorconfig.el (editorconfig--delete-trailing-whitespace):
New function.
(editorconfig-set-trailing-ws): Use it.  Use `pcase`.
Also prefer `before-save-hook` and use `add/remove-hook` to
manipulate hooks, like god intended.
Don't test `buffer-read-only` any more.
(editorconfig-mode): Don't hook into `read-only-mode-hook` any more.

* ert-tests/editorconfig-tests.el (test-trim-trailing-ws): Adjust the
test accordingly.

Match relative file names against the glob pattern instead of
trying to construct a glob pattern that matches the absolute
file name (where the code failed to escape the possible
special chars).

* lisp/editorconfig-core-handle.el
(editorconfig-core-handle-section-get-properties): Delete `dir` arg.
(editorconfig-core-handle-get-properties-hash)
(editorconfig-core-handle-get-properties): Adjust call accordingly.
Use `declare` to mark it obsolete.
(editorconfig-core-handle--fnmatch-p): Delete `dir` arg.