Add input_switch()

[cpsievert]

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[CLAassistant]

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[gadenbuie]

I left a few small comments and suggestions below.

I wonder if we should include arguments for disabled and reverse (to disable the input and to reverse the order of the label and input). I certainly see a lot of utility for disabled, but I think we'd also want to allow update_switch() to disable the input, which would require us to write some swtich-specific JS in bslib or to update the checkbox input binding in shiny. (I lean toward a bslib-specific approach.)

I'd be happy to pick up this PR and add both arguments.

[gadenbuie]

I wonder if we should include arguments for disabled and reverse (to disable the input and to reverse the order of the label and input). I certainly see a lot of utility for disabled, but I think we'd also want to allow update_switch() to disable the input, which would require us to write some switch-specific JS in bslib or to update the checkbox input binding in shiny. (I lean toward a bslib-specific approach.)

@cpsievert and I discussed in person: this PR will focus on feature parity with Shiny for Python and we'll tackle the above ideas in a future PR

[gadenbuie]

@cpsievert I applied my suggestions and added a small example app. We could suggest a similar app for Python.

[cpsievert]

Here's a slightly revised py-shiny version

[cpsievert]

Let's add a NEWS item then merge away!