feat(install): add --keepsrc to keep pkg/ and src/ directories (#2272)

* feat(install): add --nocleanbuild to keep pkg/ and src/ directories for
AUR packages

Providing this flag during installation of AUR packages allows for keeping
the src/ and pkg/ directories produced my makepkg. If the user wants to
delete the directories, they can either select to cleanBuild in the
cleanmenu or run the installation without the --nocleanbuild flag (yay
will only remove the directories if the package is rebuilt)

* fix(completion): simplify description for --nocleanbuild in fish

This makes the description consistent with the descriptions in the
man page, --help, and zsh completion.

* refactor(install): Rename --nocleanbuild to --keepsrc

This naming scheme is more familiar to users since it is the name of the
flag in Paru.

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Co-authored-by: jguer <me@jguer.space>

cmd.go

@@ -102,6 +102,7 @@ Permanent configuration options:
 
     --cleanafter          Remove package sources after successful install
     --nocleanafter        Do not remove package sources after successful build
+    --keepsrc             Keep pkg/ and src/ after building packages
     --bottomup            Shows AUR's packages first and then repository's
     --topdown             Shows repository's packages first and then AUR's
     --singlelineresults   List each search result on its own line