Fix reader syntaxes disrupted by newlines

[RuijieYu]

Note that I am writing this PR on a recent Emacs 30 build, whereas the min version supported for lispy is 24.3. I hope someone can test on some earlier versions.

Fixes #647. Also fixes #653 by its first commit.

Lispy would assume an implicit nil after the reader syntax, like this:

(defun foo ()
  '
  (bar baz)
  ) ;; point here, press i to format

Gets this incorrect result:

(defun foo ()
  '() (bar baz))

• lispy.el

(lispy--delete-insignificant-sexps): allow removing sexps that are
implicitly-created by lispy, and serve no purpose in the real input and output
sexp other than formatting. At the moment, just remove newline nodes.

(lispy--read-reader-syntax): helper function to avoid code duplication; converts
a reader syntax RS, e.g., "`", plus its immediately-next sexp, into an internal
representation with TAG.

(lispy--read): update handling of "#'", ",@", "'", "`", "," to make use of
lispy--read-reader-syntax.

(lispy--insert): fixed float and quasiquote handling (currently it calls
(insert (caddr sxp)), which is incorrect because insert considers it a
character; float was not problematic because this code path was never triggered:
things like 1.2 have not been converted into ly-raw float constructs anyway.

• lispy-test.el

(lispy-read-quote-newline): added 5 simple tests for each of the
addressed reader syntaxes.