feat(tools/passively): Bootstrap new Emacs package 'passively' r/3138

Passively is a tool to help people learn information via Emacs,
designed for language learning.

As of this CL, the actual implementation still lives in
//users/tazjin/russian/russian.el but I am generalising it here.

Change-Id: Iac5a8cfc78415496637a7ba5ddc4c2a1aa6bee26

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+<!-- SPDX-License-Identifier: MIT -->
+passively
+=========
+
+Passively is an Emacs Lisp library for passively learning new
+information in an Emacs instance.
+
+Passively works by displaying a random piece of information to be
+learned in the Emacs echoline whenever Emacs is idle for a set amount
+of time.
+
+It was designed to aid in language acquisition by passively displaying
+new vocabulary to learn.
+
+Passively is configured with a corpus of information (a hash table
+mapping string keys to string values) and maintains a set of terms
+that the user already learned in a file on disk.
+
+## Configuration & usage
+
+Configure passively like this:
+
+```lisp
+;; Configure the terms to learn. Each term should have a key and a
+;; string value which is displayed.
+(setq passively-learn-terms
+      (ht ("забыть" "забыть - to forget")
+          ("действительно" "действительно - indeed, really")))
+
+;; Configure a file in which passively should store its state
+;; (defaults to $user-emacs-directory/passively.el)
+(setq passively-store-state "/persist/tazjin/passively.el")
+
+;; Configure after how many seconds of idle time passively should
+;; display a new piece of information.
+;; (defaults to 4 seconds)
+(setq passively-show-after-idle-for 5)
+
+;; Once this configuration has been set up, start passively:
+(passively-enable)
+
+;; Or, if it annoys you, disable it again:
+(passively-disable)
+```
+
+These variables are registered with `customize` and may be customised
+through its interface.
+
+### Known terms
+
+Passively exposes the interactive function
+`passively-mark-last-as-known` which marks the previously displayed
+term as known. This means that it will not be included in the random
+selection anymore.
+
+### Last term
+
+Passively stores the key of the last known term in
+`passively-last-displayed`.
+
+## Installation
+
+Inside of the TVL depot, you can install passively from
+`pkgs.emacsPackages.tvlPackages.passively`. Outside of the depot, you
+can clone passively like this:
+
+    git clone https://code.tvl.fyi/depot.git:/tools/emacs-pkgs/passively.git
+
+Passively depends on `ht.el`.
+
+Feel free to contribute patches by emailing them to `depot@tazj.in`
+
+## Use-cases
+
+I'm using passively to learn Russian vocabulary. Once I've cleaned up
+my configuration for that, my Russian term list will be linked here.