Use ABAP CDS Syntax Highlighter for .acds files

[BeckerWdf]

With github-linguist/linguist#4614 Github understands "asddls" files and uses
the syntax coloring provided by
https://github.com/FreHu/abap-cds-grammar

Let's use an override for this repo so that acds files just use the
existing coloring for "asddls".
This is explained in:
https://github.com/github/linguist/blob/master/docs/overrides.md

Fixes: #371

[BeckerWdf]

The result can be seen e.g. in https://github.com/SAP/abap-file-formats/blob/coloring/file-formats/ddls/examples/z_aff_example_ddls.ddls.acds

The coloring is not perfect but we can file PRs on https://github.com/FreHu/abap-cds-grammar

[BeckerWdf]

Validation found dead links:

ERROR: 2 dead links found!
[✖] https://api.reuse.software/info/github.com/SAP/abap-file-formats → Status: 0
[✖] https://api.reuse.software/badge/github.com/SAP/abap-file-formats → Status: 0

But that's nothing I have changed.

[larshp]

I've retriggered the checks, now ok

[larshp]

lgtm

[BeckerWdf]

The result can be seen e.g. in https://github.com/SAP/abap-file-formats/blob/coloring/file-formats/ddls/examples/z_aff_example_ddls.ddls.acds

The coloring is not perfect but we can file PRs on https://github.com/FreHu/abap-cds-grammar

With FreHu/abap-cds-grammar#3 the grammar was improved. As soon as linguist makes a new release and as soon as github consumes this (we don't have control over this). Coloring should improve.