Add Fortran Language Parser Support

Summary

This PR adds Fortran language parsing support to LangChain's LanguageParser, enabling syntax-aware code splitting for Fortran source files.

Changes

• Implements FortranSegmenter class for parsing Fortran code structure
• Adds Fortran ('fortran' or 'f90') to supported languages in LanguageParser
• Updates LANGUAGE_EXTENSIONS and LANGUAGE_SEGMENTERS mappings
• Enables extraction of top-level functions, subroutines, and modules into separate documents
• Supports common Fortran file extensions (.f, .f90, .f95, etc.)

Usage

from langchain_community.document_loaders.generic import GenericLoader
from langchain_community.document_loaders.parsers import LanguageParser

loader = GenericLoader.from_filesystem(
    "./fortran_code",
    glob="**/*",
    suffixes=[".f90"],
    parser=LanguageParser(language="fortran")
)
docs = loader.load()

Testing

• Added unit tests for Fortran parser
• Verified parsing of functions, subroutines, and modules
• Tested with various Fortran dialects (F77, F90, F95, F2003)