Simplify .show_graph() by extracting functions for the raw syntax?

[etiennebacher]

Originally posted in #20631 but it may be better in a separate issue.

It seems to me that .show_graph() becomes quite complicated by returning various output types depending on the arguments (and #20631 also adds some complexity). It's also not very clear from the outside how those arguments interact. For example if I read

.show_graph(show=True, raw_output_format="mermaid", output_path="foobar")

then it's not clear to me what I should expect (the current behavior would be to ignore show and output_path).

Since to_dot() and to_mermaid() already exist (or will exist) on the Rust side, wouldn't it be better to have the following function signatures in Python?

def to_dot(
    self,
    *,
    [ optimization args ]
) -> str

def to_mermaid(
    self,
    *,
    [ optimization args ]
) -> str

def show_graph(
    self,
    *,
    format = "graphviz",
    output_path = None,
    [ optimization args ]
) -> None