change template prefix to curly brackets

Well... this PR simply changes the template declaration syntax from `# template` to `{template}`.
(at least syntax will be more consistent)

```
{template hello(name)}
    <div class="msg">
        Hello {name}!
    </div>
{/template}
```

NB: previous syntax is still supported - so current users can take the latest version with no need to change. But all documentation is also updated by the PR - this is why so many files have been updated..
NB2: for the reviewers: the code change is only in the pegjs grammar