feat: Add input_submit_textarea() and update_submit_textarea()

[cpsievert]

Related rstudio/bslib#1204

Adds input_submit_textarea(), a new input control that's akin to input_textarea(), but delays the sending of an input value until a corresponding button is pressed.

Here is a hello world example:

import time

from shiny.express import input, render, ui

ui.input_submit_textarea("text", placeholder="Enter some input...")


@render.text
def value():
    if "text" in input:
        # Simulate processing time
        time.sleep(2)
        return f"You entered: {input.text()}"
    else:
        return "Submit some input to see it here."

[karangattu]

since expect_rows() is duplicated in both InputSubmitTextarea and InputTextArea, we could make it a mixin similar to this

class _ExpectRowsAttrM:
    """A mixin class for the rows attribute."""

    def expect_rows(
        self: UiBaseP,
        value: AttrValue,
        *,
        timeout: Timeout = None,
    ) -> None:
        """
        Expect the `rows` attribute to have a specific value.

        Parameters
        ----------
        value
            The expected value of the `rows` attribute.
        timeout
            The maximum time to wait for the expectation to be fulfilled. Defaults to `None`.
        """
        _expect_attribute_to_have_value(self.loc, "rows", value=value, timeout=timeout)

so then the classes will look like

  class InputTextArea(
      _SetTextM,
      WidthContainerStyleM,
      _ExpectTextInputValueM,
      _ExpectPlaceholderAttrM,
      _ExpectAutocompleteAttrM,
      _ExpectSpellcheckAttrM,
      _ExpectRowsAttrM,
      UiWithLabel,
  ):
      """Controller for :func:`shiny.ui.input_text_area`."""
      .
      .
      .
  class InputSubmitTextarea(
      _SetTextM,
      WidthContainerStyleM,
      _ExpectTextInputValueM,
      _ExpectPlaceholderAttrM,
      _ExpectRowsAttrM,
      UiWithLabel,
  ):
      """Controller for :func:`shiny.ui.input_submit_textarea`."""