Add perspective-click event to grid

[Sam-Ogden]

Fixes #1076
This PR adds the perspective-click event to perspective-viewer-datagrid as specified in the docs.

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[sc1f]

Tested locally - looks great, works well for arbitrary pivots and filters. Thanks!

[KrishnaPG]

Came across this while facing a problem with click. The click event is not being raised. Used the below code from the [documentation)(https://perspective.finos.org/docs/md/js.html), but event is not getting triggered.

elem.addEventListener("perspective-click", function(event) {
    var config = event.detail.config;
    elem.restore(config);
});

Then came across this PR. Wondering if the click functionality is available yet. If not, could some one kindly merge this PR if this is what enables that click functionality specified in the documentation?

[texodus]

Thanks for the PR! Apologies for the delay in merging, this is a much-requested feature addition.

Following the pattern of existing perspective-* events for click is a natural addition to the existing API. Orthogonal to this PR (and also completely undocumented ..), ideally this would not be necessary, as we use the Custom Elements API to avoid needing to recreate DOM-like functionality. click is a good example of this:

const viewer = document.querySelector("perspective-viewer");
const table = document.querySelector("perspective-viewer regular-table");
viewer.addEventListener(x => {
    console.log(table.getMeta(x.target));
});

As it stands however, this is not particularly useful for a few reasons:

1. You must do this separately for each plugin type.
2. It is difficult to get the cell "config", a metadata dictionary of a single coordinate in a View.

For the latter at least, I propose getCellConfig() should move to @finos/perspective and perhaps undergo a name change.