Add php-indent.el, a CC Mode-free indentation engine ported from js.el

Extracted from the cc-mode independent php-mode work (#812) so that it
can be reviewed and land independently of the Emacs 28.1 requirement
that blocks that branch until the August release (#811).

The engine is ported from the syntax-ppss based indenter of js.el in GNU
Emacs (attributed in the file header); the JavaScript specific handling
(JSX, C preprocessor macros, array comprehensions and regexp literals)
is removed, and PHP specifics are added: -> / ?-> / :: method and
property chains, the . concatenation operator aligned to the assignment,
statement continuations, switch/case, the alternative syntax (endif and
friends) and heredoc/nowdoc bodies.  Its entry points are php-indent-line
and php-indent-region.

The file is self-contained: it only requires php.el for the php
customization group, and defines no symbol that collides with the
current php-mode.

This is purely additive: nothing sets php-indent-line as
indent-line-function yet, so the engine is inert -- the current CC Mode
based php-mode keeps indenting through cc-engine.  Registering both new
files in Eask so they are byte-compiled by CI.

Author: zonuexe

Eask

@@ -12,6 +12,8 @@
  "lisp/php.el"
  "lisp/php-complete.el"
  "lisp/php-defs.el"
+ "lisp/php-indent.el"
+ "lisp/php-keywords.el"
  "lisp/php-face.el"
  "lisp/php-format.el"
  "lisp/php-project.el"