Add other plot save formats

[timtmok]

Intent

Address #2657

Approach

Adds a format option when selecting where to save the plot. This adds a new parameter to the render request and updates the Python extension to render the other formats. Ark will have its own PR to add in that support. Until Ark supports the other formats, plot saving will always save as a PNG regardless of the chosen format.

QA Notes

Sample plot for Python

import numpy as np
import matplotlib.pyplot as plt

with plt.xkcd():
    # plt in widget mode
    # %matplotlib ipympl
    # v = np.array([1, 3, 5, 365])
    v = np.arange(365, 0, -1)
    # w = np.array([4, 6, 2, 10, 8, 1])

    fig, ax = plt.subplots()
    fig.set_facecolor('whitesmoke')
    fig.set_edgecolor('black')
    ax.set_xlabel('Day of the year')
    ax.set_ylabel('Days until New Year\'s Eve')

    plt.xlim(0, 365)
    plt.ylim(1, 365)

    plt.plot(v)
    # plt.plot(w)

Sample plot for R

library(ggplot2)
data("midwest")
ggplot(midwest, aes(x=area, y=poptotal)) + geom_point()

[timtmok]

I'm working on fixing the test failure

[seeM]

Python changes LGTM!

[seeM]

If the user gets to choose the file format in the modal, should we always use that file format regardless of the file name's extension? Or maybe we could show a validation error if those don't match? Silently defaulting to png feels like it would confuse users

[timtmok]

This part of saving does feel weird. RStudio sets the options to browse for a directory for the file, the file format, and file name. That seems like a better way to reduce user error and simplify validation.

[timtmok]

I've changed it to use a directory selector and separate file name field. The user selects the format. No need to check the extension anymore as it will be added for the user.

The overwrite check also occurs at save and the user can cancel if they do not wish to overwrite. Any errors during save due to write access or anything else is shown after saving and closing the dialog

[seeM]

That's much better!

Out of interest, I compared this with VSCode's save modal (you can get there by creating a new untitled file and then saving it). When you select a file format it updates the extension in the file name. If you use a mismatched format and extension, it shows the following error:

I wonder if we want to hook into that or if we prefer to roll our own? I don't have any personal preference. This feels like a broader Positron UX question.

[seeM]

Will we want to remove these defaults once ARK supports the format arg?

[timtmok]

Yes, that would be good to avoid the confusion of falling back to a png when the user selected something else.

[timtmok]

posit-dev/ark#311

I've opened the PR for Ark

[seeM]

LGTM

[timtmok]

@jonvanausdeln I'm updating the spec to reflect the changes in choosing the file path and format.