Add php-core.el for the dependency-free primitives

[zonuexe]

First step of the php.el decomposition, aimed at the v1.28 series.

Why

php.el is required by every PHP package, but it requires flymake, php-project and (for now) CC Mode. A module that only wants the php customization group or a syntax-ppss predicate pays for all of it — measured on a byte-compiled tree:

	features	time
(require 'php)	17	~53 ms

The indentation engine pulling in flymake to get a 7-line defgroup is the clearest symptom.

What moves

lisp/php-core.el (new, zero dependencies):

• the php customization group
• php-executable
• php-in-string-p, php-in-comment-p, php-in-string-or-comment-p, php-in-poly-php-html-mode
• php-base-mode

php.el requires php-core.el and therefore still exposes every one of them, so (require 'php) behaves exactly as before and nothing has to migrate. This is the facade direction: the dependency arrow points core ← php, with no cycle.

php-indent.el, which only ever wanted the customization group, now requires php-core.el instead:

	features	time
(require 'php-indent)	2	~1 ms

with no flymake, php-project or cc-engine in its chain.

Two consequences worth calling out

php.el's defcustoms specify :group 'php again. #813 removed those as redundant, which was true only while the defgroup sat in the same file: custom-current-group keys off load-file-name, so with the defgroup in php-core.el they silently fall out of the group. Without this, M-x customize-group php lists 2 members instead of 29. They are required now, not redundant.

The syntax tables stay in php.el for the moment. They are built with c-populate-syntax-table, so moving them would drag CC Mode into php-core.el and defeat the purpose. They can follow once php.el stops requiring cc-engine.

Verification

• (get GROUP 'custom-group) membership is identical to master for every php* group (php, php-mode, php-faces, php-format, php-ide, php-ide-phpactor, php-flymake, php-complete, php-project).
• Byte-compiles with no new warnings; the existing test suite is unaffected (62 tests, unchanged).
• php-core.el uses no Emacs 28+ functions, so it does not front-run Drop Emacs 27 Support #811 — this is intended for the Emacs 27-supporting v1.28 series.

Follow-ups (not here)

• php-executable currently has two bound-and-true-p / boundp workarounds — php-project.el:224 and php-flymake.el:104 — because neither can require php.el (php.el requires php-project, so it would be circular). Both can now require php-core.el instead and drop the workaround. Kept out of this PR to keep it a pure move with no behaviour change.
• Moving the syntax tables and the token scanner, once cc-engine is gone from php.el.
• php-magical-constants → php-keywords.el with an obsolete alias.