Fix lexing of numbers with trailing decimal point and exponent

Fix lexing of numbers with trailing decimal point and exponent

Numbers with a trailing dot and an exponent were incorrectly read as
integers (with the exponent ignored) instead of the floats they should
be.  For example, 1.e6 was read as the integer 1, not 1000000.0 as
every sane person would agree was meant. (Bug#48678)

Numbers with a trailing dot but no exponent are still read as
integers.

* src/lread.c (string_to_number): Fix float lexing.
* test/src/lread-tests.el (lread-float): Add test.
* doc/lispref/numbers.texi (Float Basics): Clarify syntax.