This adds -std=f2023, which is mostly a prep patch for future changes.

However, Fortran 2023, https://j3-fortran.org/doc/year/23/23-007r1.pdf changes two things which is taken
care in this patch:

(A) In "6.3.2.1 Free form line length":

Fortran 2018: "If a line consists entirely of characters of default kind (7.4.4), it shall contain at most 132 characters"
Fortran 2023: "A line shall contain at most ten thousand characters."

(B) In "6.3.2.6 Free form statements":
Fortran 2018: "A statement shall not have more than 255 continuation lines."
Fortran 2023: "A statement shall not have more than one million characters."

I have not added a testcase for exceeding the latter but otherwise there are new
tests and I had to add a couple of -std=f2018 to existing tests.

Comments, suggestions, approval?

Tobias

PS: I find it funny that -std=c23, -std=c++23 and -std=f2023 will get added in the same GCC release.

PPS: I did not bother adding .f23 as file extension; I believe that also .f18 is unsupported.
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