low: flow.filter shortcut

[Slava Bereza]

    signal
      .from(() => sharedCardPage().hasCard)
      .flow.filter((flag) => flag)
      .watch(this.inc)


// -->

  .flow.filter() // same as  .flow.filter((flag) => flag)
  .flow.filter.not() //  same as .flow.filter((flag) => !flag)

[Slava Bereza]

Implemented for "filter" and "filter.not"
0.6.0