[Python] Add Pythonic import path syntax for relative imports

[dbrattli]

This PR adds support for Python-style relative import syntax in importMember, import, and importAll functions, making Python interop more idiomatic for Python developers. Wanted to this fix for a long time.

Changes

New Pythonic syntax for relative imports:

• .module → resolves to module in current directory (like from .module import ...)
• ..module → resolves to module in parent directory (like from ..module import ...)
• ...module → resolves to module in grandparent directory, and so on

Examples:

// Current directory
let add5 (x: int): int = importMember ".native_code"
// Generates: from tests.Python.native_code import add5

// Parent directory  
let foo: int -> int = importMember "..utils"
// Generates: from tests.utils import foo

// Subpackage
let bar: string = importMember ".subpkg.helpers"
// Generates: from tests.Python.subpkg.helpers import bar

// External packages (unchanged - pass through as-is)
let Field: obj = importMember "pydantic"
// Generates: from pydantic import Field

Backwards compatibility:

• File path syntax (./native_code.py) still works
• Absolute package imports ("pydantic", "collections.abc") work unchanged

Files Changed

• src/Fable.Cli/Pipeline.fs - Convert Python-style paths to file paths for resolution
• src/Fable.Transforms/Python/Fable2Python.Transforms.fs - Handle identifier extraction for relative imports
• src/Fable.Core/Fable.Core.PyInterop.fs - Updated documentation
• tests/Python/TestPyInterop.fs - Updated tests to use new syntax

Motivation

The previous JS-style syntax ("./native_code.py") was not idiomatic for Python. Python developers expect module imports to use dot notation without file extensions. This change aligns Fable's Python import syntax with Python's native import conventions.