Simplify macros, support LFortran, and expand testing

Breaking changes:

This release removes the assert subroutine's diagnostic_data argument and related features and documentation, including

• The characterizable_t and intrinsic_t derived types and
• The call_assert_diagnostic macro.

To produce rich diagnostic messages from failed assertions, please see the Julienne framework. Julienne wraps Assert and defines correctness-checking idioms that automatically generate diagnostic messages containing program data. Alternatively, Julienne offers string-handling utilities that support user-customizable diagnostic messages.

Additional changes

• Support the LFortran compiler,
• Expand the CI testing matrix,
• Remove the FORD documentation, which Assert's simplified interface renders superfluous, and
• Update Assert's README.md to reflect the above modifications.

Pull Request Log

• CI expansion: Add macos-15, flang-20 on macos and flang-latest on Linux by @bonachea in #62
• Support LFortran and simplify assertions by @rouson in #61
• Simplify macro by @bonachea in #63
• Remove intermediate submodule by @bonachea in #64
• Disable FORD generation of documentation by @bonachea in #66
• doc(README): rm ford, update intro by @rouson in #65

Full Changelog: 2.1.2...3.0.0