Make converter compatible with bem-xjst 4+

[vkz]

see https://github.yandex-team.ru/lego/islands/pull/509 Compatibility with the latest bem-xjst should be available with relevant flag in the API

• [x] assert that every template has sub-predicate matching block
• [x] assert there're no predicates matching this.elem https://github.com/bem/bem-xjst/issues/89
• [x] assert that every apply sets the mode as its first argument https://github.com/bem/bem-xjst/issues/91
• [x] warn when apply assigns block, elem, mods, elemMods https://github.com/bem/bem-xjst/issues/73
• [x] convert def() + applyCtx with this.ctx inside into wrap() see article7 in https://github.yandex-team.ru/lego/islands/pull/509
• [x] ensure there's always return in def mode. Insert return '' when there is none
• [x] assert this._buf is never used in templates

[vkz]

apply() checks test:

  block b1 {
    default: { apply({test: 42, elemMods: 'bla'});}
  }