Send JS command values on phx-submit

[SteffenDE]

Alternative implementation for #3674.

[jotaviobiondo]

For the phx-value-* and JS.push/2 :value attributes, should we put inside a value or values key in meta?

Example:

<form phx-submit={JS.push("...", value: %{js_push_value: 1})} phx-value-attribute="2"></form>

# would have this meta:

meta: {
  _target: ...,
  js_push_value: 1,
  attribute: "2"
}

# but maybe it should be

meta: {
  _target: ...,
  value: {
    js_push_value: 1,
    attribute: "2"
  }
}

To not mix these values with possibly other meta keys that the submit might have.
What do you think?

[SteffenDE]

I don't think it matters, because to keep the current behavior, either way values could be overridden somewhere.

E.g. you could do JS.push("..."; value: %{_target: "other target"}) and even with this change, on the server when we merge the values, the "other target" would win.

[jotaviobiondo]

Hmmm got it!

so for me it looks good! :)

[josevalim]

LGTM we just need to make sure we are consistent. Who wins between JS.push, phx-value and the form inputs? We need to make sure this is documented too.