Add modern Fortran comment style

The current comment style for Fortran seems to target fixed form source code, where any character in col.1 would start a comment. The character usually is "c" or ''C".
Free form source code, on the other hand, requires an "!" to start comments. Adding comments with a leading "c" is not supported if I'm not mistaken, sources would no longer compile, `reuse` would thus break the code.

This PR adds an additional comment style for modern free form Fortran and adjusts the selection to commonly used file extensions. While the easiest fix would have been to replace
```python
    SINGLE_LINE = "c"
```
with
```python
    SINGLE_LINE = "!"
```
I think it is more customary to have 'c' with fixed form source code, hence the additional style block.

Signed-off-by: Carmen Bianca BAKKER <carmenbianca@fsfe.org>

AUTHORS.rst

@@ -66,6 +66,8 @@ Contributors
 
 - Shun Sakai
 
+-  Dirk Brömmel
+
 Translators
 -----------
 

CHANGELOG.md

@@ -44,6 +44,7 @@ CLI command and its behaviour. There are no guarantees of stability for the
 
 - More file types are recognised:
   - Julia (`.jl`) (#815)
+  - Modern Fortran (`.f90`) (#836)
 
 ### Changed
 

src/reuse/comment.py

@@ -14,6 +14,7 @@
 # SPDX-FileCopyrightText: 2023 Kevin Meagher
 # SPDX-FileCopyrightText: 2023 Mathias Dannesbo <md@magenta.dk>
 # SPDX-FileCopyrightText: 2023 Shun Sakai <sorairolake@protonmail.ch>
+# SPDX-FileCopyrightText: 2023 Juelich Supercomputing Centre, Forschungszentrum Juelich GmbH
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
@@ -341,14 +342,23 @@ def comment_at_first_character(cls, text: str) -> str:
 
 
 class FortranCommentStyle(CommentStyle):
-    """Fortran comment style."""
+    """Fortran (fixed form) comment style."""
 
     SHORTHAND = "f"
 
     SINGLE_LINE = "c"
     INDENT_AFTER_SINGLE = " "
 
 
+class ModernFortranCommentStyle(CommentStyle):
+    """Fortran (free form) comment style."""
+
+    SHORTHAND = "f90"
+
+    SINGLE_LINE = "!"
+    INDENT_AFTER_SINGLE = " "
+
+
 class FtlCommentStyle(CommentStyle):
     """FreeMarker Template Language comment style."""
 
@@ -562,15 +572,18 @@ class XQueryCommentStyle(CommentStyle):
     ".ex": PythonCommentStyle,
     ".exs": PythonCommentStyle,
     ".f": FortranCommentStyle,
-    ".f03": FortranCommentStyle,
-    ".f90": FortranCommentStyle,
-    ".f95": FortranCommentStyle,
+    ".f03": ModernFortranCommentStyle,
+    ".f08": ModernFortranCommentStyle,
+    ".f90": ModernFortranCommentStyle,
+    ".f95": ModernFortranCommentStyle,
     ".fish": PythonCommentStyle,
     ".fnl": LispCommentStyle,
     ".fodp": UncommentableCommentStyle,
     ".fods": UncommentableCommentStyle,
     ".fodt": UncommentableCommentStyle,
     ".for": FortranCommentStyle,
+    ".ftn": FortranCommentStyle,
+    ".fpp": FortranCommentStyle,
     ".fs": CCommentStyle,
     ".ftl": FtlCommentStyle,
     ".gemspec": PythonCommentStyle,