elec-pair: Fix bug incorrectly hiding syntax-propertize-function

elec-pair: Fix bug incorrectly hiding syntax-propertize-function

* lisp/elec-pair.el (electric-pair--with-text-syntax): New macro.
(electric-pair-syntax-info):
(electric-pair--balance-info):
(electric-pair--syntax-ppss, electric-pair--balance-info): Use it.
(electric-pair--with-uncached-syntax): Remove (bug#49629).

The main bug that this is fixing is `syntax-propertize-function' being
hidden in `electric-pair--balance-info' when the original syntax table
is to be used, not `electric-pair-text-syntax-table'.

Notably, this causes `electric-pair-mode' to often misbehave in HTML
files when pairing angle brackets.

This commit also flushes the cache before installing
`electric-pair-text-syntax-table', to prevent cached syntax for the
original table from affecting things.