feat: Consistent and sensible layout_config (#922)

* feat: Consistent and sensible layout_config

* [docgen] Update doc/telescope.txt
skip-checks: true

* [WIP]: Thu 17 Jun 2021 03:36:44 PM EDT

* [WIP]: Thu 17 Jun 2021 03:38:11 PM EDT

* layout_default -> layout_defaults

* remove options from bug repot

* Conni2461 suggestions: part 1

* [docgen] Update doc/telescope.txt
skip-checks: true

* Conni2461 suggestions: part 2

* [docgen] Update doc/telescope.txt
skip-checks: true

* Linting

* Improve deprecation checks

- Move `layout_defaults` handling to `deprecated.lua`
- Check for "layout keys" outside of `layout_config` on `setup`

* fixup: Just add a few more words

Co-authored-by: Luke Kershaw <35707277+l-kershaw@users.noreply.github.com>
Co-authored-by: Github Actions <actions@github>

lua/telescope/previewers/init.lua

@@ -302,7 +302,6 @@ previewers.git_commit_message        = buffer_previewer.git_commit_message
 --- The run command is `git --no-pager diff $FILE`
 previewers.git_file_diff             = buffer_previewer.git_file_diff
 
-
 previewers.ctags = buffer_previewer.ctags
 previewers.builtin = buffer_previewer.builtin
 previewers.help = buffer_previewer.help