fix: enable linting changed files with textlint

textlint was expensive to run because we added every file in the list of
files to lint to FILE_ARRAY_NATURAL_LANGUAGE. In #5041, we mitigated
this issue but lost the ability to run textlint on changed files only.
Given that textlint ignore files for which it doesn't have a plugin
installed, and that we don't currently install additional plugins
besides the default ones to lint markdown files and text files, we let
textlint run on these files only, so we can have the feature to lint
only changed files with this linter as well, back.