Anchor inline regexps and improve sub/super-script

The regexps are meant to extract a match immediately following the cursor - the
anchor should have been there from the beginning...

Also empty sub/superscript doesn't make sense - nested sub/superscript does
make sense but yagni.

org/testdata/inline.html

@@ -61,7 +61,7 @@ empty emphasis markers like ++ // __ and so on are ignored
 </li>
 <li>
 <p>
-subscript<sub>sub</sub> and superscript<sup>super</sup>
+use _{} for subscript<sub>sub</sub> and ^{} for superscript<sup>super</sup>
 </p>
 </li>
 <li>

org/testdata/inline.org

@@ -17,7 +17,7 @@
   is
   not emphasized/
 - empty emphasis markers like ++ // __ and so on are ignored
-- subscript_{sub} and superscript^{super}
+- use _{} for subscript_{sub} and ^{} for superscript^{super}
 - links
   1. regular link [[https://example.com]] link without description
   2. regular link [[https://example.com][example.com]] link with description

org/testdata/inline.pretty_org

@@ -17,7 +17,7 @@
   is
   not emphasized/
 - empty emphasis markers like ++ // __ and so on are ignored
-- subscript_{sub} and superscript^{super}
+- use _{} for subscript_{sub} and ^{} for superscript^{super}
 - links
   1. regular link [[https://example.com]] link without description
   2. regular link [[https://example.com][example.com]] link with description