Emacs: notmuch-indicator version 0.2.0

This is a simple package that renders an indicator with an email count of the notmuch index on the Emacs mode line. The underlying mechanism is that of notmuch-count(1), which is used to find the number of items that match the given search terms. In practice, the user can define one or more searches and display their counters.

Package name (GNU ELPA): notmuch-indicator
Git repo on SourceHut: https://git.sr.ht/~protesilaos/notmuch-indicator
- Mirrors:
  - GitHub: https://github.com/protesilaos/notmuch-indicator
  - GitLab: https://gitlab.com/protesilaos/notmuch-indicator
Mailing list: https://lists.sr.ht/~protesilaos/notmuch-indicator
Backronym: notmuch-… Increasingly in Need of Displaying Inconsequential Counters Alongside Trivia that Obscure Reality.

Below are the release notes.

Fixed how the indicator is added to global-mode-string. Before, it was inserted as a string. Whereas we want to pass it as a symbol. This is how other modes do it as well. The result is the same for the user, though this design ensures that we do not get false positives when we try to remove our indicator (e.g. if we target an empty string, we can delete something else).

Thanks to Yusef Aslam for reporting a bug which revealed this problem. This was done in issue 1 over at the GitHub mirror: https://github.com/protesilaos/notmuch-indicator/issues/1.
Made the indicator’s counters clickable. The mouse-1 will now use the terms that yield the individual message counter to run a notmuch-search with them. Thanks to Henrik Kjerringvåg for the patch. It is below the ~15 line threshold and thus requires no copyright assignment to the Free Software Foundation.

As a reminder, the search terms are specified in the user option notmuch-indicator-args and are passed to notmuch-count(1) which yields the count. For instance, a value like the following defines three counters, which likely result in @10 😱9999 💕0:
```
(setq notmuch-indicator-args
      '((:terms "tag:unread and tag:inbox" :label "@")
        (:terms "from:bank and tag:bills" :label "😱")
        (:terms "--output threads tag:loveletter" :label "💕")))
```
Implemented some minor refinements to the code, based on the feedback of Stefan Monnier on the emacs-devel mailing list: https://lists.gnu.org/archive/html/emacs-devel/2022-09/msg01949.html.

The package is in a stable state and will likely not need much maintenance going forward. Consult the package’s README or the Commentary section of notmuch-indicator.el for the technicalities. The latter is displayed in the Help buffer when using M-x describe-package for notmuch-indicator.