Implement list-modes command

[bendiksolheim]

Adds a new command list-modes which lists modes currently known to AeroSpace. This is to solve issue #377. I have not added support for formats, JSON, or anything like that.

I am not sure if I have found every file to update with a new command (like docs, and so on), so just tell me if I need to add it somewhere else.

[nikitabobko]

Thanks, it's a good-quality PR. Some contributors don't even bother to search for files that needs to be updated, you did great, you found all of them.

I tried to auto-generate most of them, so they couldn't be forgotten, there are only two which require manual adjustment: ./docs/commands.adoc, grammar/commands-bnf-grammar.txt. The first one can be probably auto-generated as well. The second one is much harder to auto-generate.

I have done 2 minor adjustments to the PR:

1. I squashed all commits together
2. I did a minor fix in the docs, see the patch below.

the patch:

diff --git a/docs/aerospace-list-modes.adoc b/docs/aerospace-list-modes.adoc
index b36571a2..9bdcb048 100644
--- a/docs/aerospace-list-modes.adoc
+++ b/docs/aerospace-list-modes.adoc
@@ -20,7 +20,6 @@ aerospace list-modes [-h|--help] [--current]
 {manpurpose}
 
 See xref:guide.adoc#binding-modes[the guide] for documentation about binding modes
-// end::body[]
 
 // =========================================================== Options
 include::util/conditional-options-header.adoc[]
@@ -30,5 +29,7 @@ include::util/conditional-options-header.adoc[]
 --current::
 Only print the currently active mode
 
+// end::body[]
+
 // =========================================================== Footer
 include::util/man-footer.adoc[]

I have not added support for formats, JSON, or anything like that.

It's a good point, thanks for mentioning it. I've created an issue: #1026 If you feel adventurous, feel free to pick it up.