Let kwargs_to_strings work with default values and positional arguments

[seisman]

Description of proposed changes

See #2361 for the original issue report. The solution comes from https://stackoverflow.com/a/69170441.

In summary, it's possible to use the following codes to get the default values of parameters:

sig = inspect.signature(f)
bound = sig.bind(*args, **kwds)
bound.apply_defaults()

Here are some example codes to understand the behavior the the above codes, in which case 3 is usually how we call PyGMT functions/methods:

import inspect

def func(x, y, z=3, **kwargs):
    """
    An example function which 
    """
    print(f"x={x} y={y} z={z}")
    print(kwargs)

print("Case 1:")
sig = inspect.signature(func)
bound = sig.bind(1, 2)
print(bound.arguments)  # raw arguments
bound.apply_defaults()
print(bound.arguments)  # after apply_defaults

print("Case 2:")
bound = sig.bind(1, 2, 4, R=[1, 2, 3, 4], J="X10c")
bound.apply_defaults()
print(bound.arguments)

print("Case 3:")
bound = sig.bind(1, 2, z=4, R=[1, 2, 3, 4], J="X10c")
bound.apply_defaults()
print(bound.arguments)

bound.arguments["kwargs"]["R"] = "1/2/3/4"
print(bound.arguments)
print(bound.args, bound.kwargs)

func(*bound.args, **bound.kwargs)

The outputs are:

Case 1:
{'x': 1, 'y': 2}
{'x': 1, 'y': 2, 'z': 3, 'kwargs': {}}
Case 2:
{'x': 1, 'y': 2, 'z': 4, 'kwargs': {'R': [1, 2, 3, 4], 'J': 'X10c'}}
Case 3:
{'x': 1, 'y': 2, 'z': 4, 'kwargs': {'R': [1, 2, 3, 4], 'J': 'X10c'}}
{'x': 1, 'y': 2, 'z': 4, 'kwargs': {'R': '1/2/3/4', 'J': 'X10c'}}
(1, 2, 4) {'R': '1/2/3/4', 'J': 'X10c'}
x=1 y=2 z=4
{'R': '1/2/3/4', 'J': 'X10c'}

This PR will fix #2361. With changes in this PR, now we don't need to manually deal with offset in Figure.timestamp (see
#2208), panel in subplot (3edb1ee) and fname in pygmt.which (#2726).

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

I've merged this PR without review and approval so that I can work on other issues that need the changes in this PR.