Create a standalone tf-runs-selector

This change creates a `tf-runs-selector` component for selecting multiple
runs, and implements it on the scalars dashboard as proof of concept. It
fulfills the same functionality as the existing `tf-run-selector` (note
the singular), but with a different and simpler API: it takes no input
and produces as output a list of selected runs. It uses a centralized
runs store as its source of truth; this will facilitate synchronizing
all run selectors to the same set of runs.

It currently behaves the same way as `tf-run-selector` with the
exception of the order of runs: this new component sorts the runs, while
the old component did not (so it was getting them in arbitrary order
from the `/runs` endpoint). It seems like sorting them is a good
idea---prefer determinism unless there's a good reason---but this will
result in a bit of drift among plugins with the old and new selectors in
this transition period. To combat that, I've made the color scale sort
its domain, so that at least the colors are consistent. (I don't see a
reason not to keep this in the final version, either.)

PiperOrigin-RevId: 158898890