tildify: add `tildify-double-space-undos'

tildify: add `tildify-double-space-undos'

* lisp/textmodes/tildify.el (tildify-double-space-undos): A new
variable specifying whether pressing space in `tildify-mode' after
a space has been replaced with hard space undos the substitution.
(tildify-space): Add code branch for handling `tildify-doule-space'.

* tests/automated/tildify-tests.el (tildify-space-undo-test--test):
A new helper function for testing `tildify-double-space-undos'
behaviour in the `tildify-space' function.
(tildify-space-undo-test-html, tildify-space-undo-test-html-nbsp)
(tildify-space-undo-test-xml, tildify-space-undo-test-tex): New
tests for `tildify-doule-space-undos' behaviour.