CODEOWNERS: Use sections to allow multiple matching entries

The CODEOWNERS documentation has this to say on the current matching
behaviour:

> The path definition order is significant: the last pattern matching a
> given path is used to find the code owners.

Take this as an example:

    /rts/        bgamari [...]
    /rts/win32/  Phyx

(I'm omitting the '@' so as to not notification spam everyone)

This means a change in a file under win23 would only have Phyx but not
bgamari as approver. I don't think that's the behaviour we want.

Using "sections" we can get additive behaviour instead, from the docs:

> Additionally, the usual guidance that only the last pattern matching the
> file is applied is expanded such that the last pattern matching for each
> section is applied.

    [RTS]
    /rts/        bgamari [...]

    [WinIO]
    /rts/win32/  Phyx

So now since those entries are in different sections both would be added to
the approvers list.

The sections feature was introduced in Gitlab 13.2, see "Version history"
on [1] we're currently running 18.8 on gitlab.haskell.org, see [2].

[1]: https://docs.gitlab.com/13.8/ee/user/project/code_owners.html#code-owners-sections
[2]: https://gitlab.haskell.org/help

CODEOWNERS

@@ -4,18 +4,18 @@
 # Catch-all
 * @bgamari
 
-# Build system
+[Build system]
 /hadrian/                         @snowleopard @alp @DavidEichmann @angerman
 /configure.ac                    @angerman @hvr
 /aclocal.m4                      @angerman @hvr
 /config.sub                      @angerman @hvr
 
-# RTS-like things
+[RTS-like things]
 /rts/                             @bgamari @simonmar @osa1 @Phyx @angerman
 /rts/linker/                      @angerman @Phyx @simonmar
 /includes/                        @bgamari @simonmar @osa1
 
-# The compiler
+[The compiler]
 /compiler/parser/                  @int-index
 /compiler/GHC/Hs/                  @simonpj @rae
 /compiler/GHC/Tc/                  @simonpj @rae
@@ -37,19 +37,19 @@
 /compiler/GHC/Core/Op/DmdAnal.hs   @simonpj @sgraf
 /compiler/GHC/ThToHs.hs            @rae
 
-# Core libraries
+[Core libraries]
 /libraries/base/                  @hvr
 /libraries/ghci/                  @simonmar
 /libraries/template-haskell/      @rae
 
-# Internal utilities and libraries
+[Internal utilities and libraries]
 /libraries/libiserv/              @angerman @simonmar
 /utils/iserv-proxy/               @angerman @simonmar
 /utils/iserv/                     @angerman @simonmar
 /utils/gen-dll/                   @Phyx
 /utils/fs/                        @Phyx
 
-# WinIO related code
+[WinIO related code]
 /libraries/base/GHC/Event/Windows/                   @Phyx
 /libraries/base/GHC/IO/Windows/                      @Phyx
 /rts/win32/                                          @Phyx