Web.JSON: Add JSON special tokens support

Web.JSON.encode({val}) function support JSON special tokens (true/false/null).
If these tokens are specified, the function return 'true', 'false', or
'null' (not '"true"', '"false"', or '"null"').
You can access to the corresponding tokens as Web.JSON.true,
Web.JSON.false, and Web.JSON.null respectively.

    let s:JSON = s:V.import('Web.JSON')
    " the following echo '[true, false, null]'
    echo s:JSON.encode([
            \ s:JSON.true,
            \ s:JSON.false,
            \ s:JSON.null,
            \])

Notice that for backward compatibility, these special tokens are converted to
1, 0, and 0 respectively in `Web.JSON.decode(val)` function like

    " the following echo '[1, 0, 0]'
    echo s:JSON.decode(s:JSON.encode([
            \ s:JSON.true,
            \ s:JSON.false,
            \ s:JSON.null,
            \]))
Web.JSON: Update Changes