Update the typer[all] dependency to typer-slim[standard] (#1287)

In 0.12.1, Typer was significantly reorganized.

- `typer-slim` is the library (for `import typer`)
- `typer-slim[standard]` adds optional dependencies (currently `rich`
  and `shellingham`, basically equivalent to the old `typer[all]`)
- `typer-cli` is the `typer` command-line tool
- `typer` is now basically a metapackage that brings in *all of the
  above*, and it no longer has an `all` extra

Pip will warn about this and proceed,

```
WARNING: typer 0.12.1 does not provide the extra 'all'
```

but there are other tools that will fail hard when asked to resolve a
(now) nonexistent extra.

Since this project doesn’t need the `typer` command-line tool, it looks
like changing the dependency to `typer-slim[standard]` is the best way
forward.

See https://typer.tiangolo.com/release-notes/#0121 and
tiangolo/typer#785 for further discussion
and details.

pyproject.toml

@@ -60,7 +60,7 @@ test = [
   "types-Pillow",
   "types-humanfriendly",
 ]
-watcher = ["watchdog>=1.0.2", "typer[all]", "python-dotenv"]
+watcher = ["watchdog>=1.0.2", "typer-slim[standard]", "python-dotenv"]
 webservice = ["Flask>=2.0.1"]
 
 [project.scripts]