Move syscall-args and is-syscall to gef-extras

[hugsy]

Description/Motivation/Screenshots

In the purpose of continuing moving everything that has a dependency (Python modules or external files) to GEF-Extras, this PR removes the 2 commands syscall-args and is-syscall from GEF itself.

Docs & tests were moved as well.

See :

• Add syscall-args and is-syscall gef-extras#74

Against which architecture was this tested ?

"Tested" indicates that the PR works and the unit test (i.e. make test) run passes without issue.

• x86-32
• x86-64
• ARM
• AARCH64
• MIPS
• POWERPC
• SPARC
• RISC-V

Checklist

• My PR was done against the dev branch, not main.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.