feat(#3119): Expose filter functions to the api

[gracepetryk]

Addresses #3119. Filter methods have all been normalized to accept a single path variable so that they may be easily called during a call to custom. Results are cached based on the path argument during each render loop to avoid computing filters more than once if the filter is enabled and also called in custom.

This enables filter configurations that were not possible before, for example combining filters so that items are only filtered if they match both:

require('nvim-tree').setup({
  filters = {
    custom = function (path)
      local filter_api = require('nvim-tree.api').filters

      return filter_api.git_clean(path) and filter_api.no_buffer(path)
    end
  }
})