Add a Fortran syntax highlighter

[tiresiasfromthebai]

Why

I've been testing MPI debugging with Seer (#60 is on my radar): half the
MPI world writes Fortran, and Seer had no highlighter for it — Fortran
sources show up as plain text.

What

A SeerFortranSourceHighlighter for free-form Fortran (90 and later),
modeled on the Ada one (the other case-insensitive language), plus the
usual plumbing: suffix list in the settings (defaults to
.f90|.F90|.f95|.F95|.f03|.F03|.f08|.F08), a Fortran tab in the editor
config page, save/restore of the suffixes, dispatch in the highlighter
factory, CMakeLists.

Fortran-specific choices, for review:

• Comments are ! only. Note // is the string concatenation operator
  in Fortran, not a comment — easy trap coming from C.
• Keywords are matched case-insensitively (like Ada). The list covers
  free-form Fortran 90→2008 keywords plus the dotted operators and
  constants (.and., .eq., .true., …).
• The symbolic operators (==, /=, <=, >=, <, >, //, =>)
  use the keyword format too, so == renders like its word form .eq..
  This is safe in Fortran (no templates — </> are always relational).
• Strings match with both delimiters ("..." and '...').
• No generic "function" rule: name(...) is also how Fortran references
  arrays, so a before-parenthesis rule would mislabel half the
  identifiers. Instead, a closed list of the common intrinsics
  (trim, sqrt, size, matmul, …) gets the function format when
  followed by (. A nice side effect: len/kind stay keywords in
  declarations (character(len=32)) and render as functions in calls.
• No "class" rule: the language is case insensitive with no reliable case
  convention; derived types are already caught by the type/class
  keywords.
• Fixed-form Fortran (F77 .f/.for, comment in column 1, code starting
  column 7) is out of scope — it would need column-aware rules and the
  legacy dialect is rarely what people debug interactively.

One small extra

tests/hellofortran90/Makefile hardcoded FC = gfortran-12, so the test
didn't build on machines with any other gfortran (mine has 13). Changed
it to plain gfortran — that also makes it easy to try this highlighter
on that test.

Testing

Qt 6.4.2, Linux Mint, gfortran 13. Verified on tests/hellofortran90
(which now builds out of the box, see above) and on a dedicated test
sheet exercising the traps above (mixed-case keywords, // concat not
colored as comment, both string delimiters, dotted operators).
Screenshot below. The other languages' highlighting is untouched (the
factory just gains one branch).

[epasveer]

Thanks for this. I'll review it in the next couple days.