Add support for conditional code

[javier-godoy]

Lines with a // hide-source comment are not rendered in the code snippet (#37).
Lines with a // show-source comment are rendered in the code snippet (#42).

What we need now is a way to display "nice" code depending on the framework version.

For instance:

// #if vaadin ge 22
// show-source foo();
// #elif vaadin eq 23
// show-source bar();
// #else
// show-source baz();
// #endif

would be rendered as foo(); in Vaadin 14-22, bar in Vaadin 23 and baz() in Vaadin 24.