libutil/logging: extend internal-json logger to make it more machine-readable

The new error-format is pretty nice from a UX point-of-view, however
it's fairly hard to parse the output e.g. for editor plugins such as
vim-ale[1] that use `nix-instantiate --parse` to determine syntax errors in
Nix expression files.

This patch extends the `internal-json` logger by adding the fields
`line`, `column` and `file` to easily locate an error in a file and the
field `raw_msg` which contains the error-message itself without
code-lines and additional helpers.

An exemplary output may look like this:

```
[nix-shell]$ ./inst/bin/nix-instantiate ~/test.nix --log-format minimal
{"action":"msg","column":1,"file":"/home/ma27/test.nix","level":0,"line":4,"raw_msg":"syntax error, unexpected IF, expecting $end","msg":"<full error-msg with code-lines etc>"}
```

[1] https://github.com/dense-analysis/ale

doc/manual/command-ref/opt-common.xml

@@ -106,7 +106,14 @@
     </varlistentry>
 
     <varlistentry><term>internal-json</term>
-    <listitem><para>Outputs the logs in a structured manner. NOTE: the json schema is not guarantees to be stable between releases.</para></listitem>
+     <listitem>
+      <para>Outputs the logs in a structured manner.</para>
+      <warning>
+       <para>
+        While the schema itself is relatively stable, the format of the error-messages (namely of the <literal>msg</literal>-field) can change between several releases.
+       </para>
+      </warning>
+     </listitem>
     </varlistentry>
 
     <varlistentry><term>bar</term>