Facilitate annotating `plot_coefs`

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I'd like to make it possible for users to put written annotations on `plot_coefs`/`plot_summs` output. I'm thinking about putting the slope, maybe test statistic, standard error, p-value, confidence interval in any combination.

My first thought is to use `glue` and let users customize by providing a string a la `huxtable::huxreg`. So as a default the string would be something like `"{estimate} ({standard.error}), t = {statistic}"`. That part will be easier to implement than the actual plotting of the text.

Start by locating the plot_coefs and plot_summs entry points and reviewing how their output is assembled; the issue names glue and huxtable::huxreg as references for customizable annotation strings. Done means users can choose which statistics to display and see annotations on both plotting outputs, with the proposed default format supported.

Written by the indexing model from the issue text.