Set the default PolkaVM stack and heap size values to 128KB. (#441)

Fixes https://github.com/paritytech/contract-issues/issues/149

A few test cases from v2-periphery-polkadot are hitting the PVM stack
limit when the contracts are compiled using resolc version 0.5.0. The
same tests pass if the contracts are compiled with a resolc version that
includes commit:
https://github.com/paritytech/revive/commit/878ca91689b99220f7ffd40df760613b7314de86.

After a brief discussion with @xermicus, we propose to set the default
PolkaVM stack and heap size values to 128KB, since we have already
encountered tests that hit the stack size limit and, without real world
data, it is difficult to determine the optimal values.

docs/developer_guide/target.html

@@ -35,7 +35,7 @@
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             const default_light_theme = "light";
             const default_dark_theme = "navy";
-            window.path_to_searchindex_js = "../searchindex-82723808.js";
+            window.path_to_searchindex_js = "../searchindex-4c7bbe7b.js";
         </script>
         <!-- Start loading toc.js asap -->
         <script src="../toc-6f3f265e.js"></script>