Add ABAP extension

[zooloo303]

Adds an ABAP + ABAP CDS language extension.

• Languages: ABAP (.abap, incl. abapGit .clas.abap) and ABAP CDS (.asddls, .asddlx).
• Grammars: two MIT tree-sitter grammars I wrote — tree-sitter-abap and tree-sitter-abap-cds — for highlighting, outline, brackets, indentation.
• Language server: thin stdio wrapper around @abaplint/core (MIT) for diagnostics, hover, go-to-definition, references, rename, formatting. The server is not shipped in the extension — it's downloaded from the extension repo's GitHub releases at runtime, per the guidelines.
• ID abap contains no zed/extension; MIT-licensed; no theme bundled.

Testing: installed as a dev extension in Zed and verified on real abapGit-serialized sources (classes + CDS views) — highlighting, outline, and inline abaplint diagnostics all working for both ABAP and ABAP CDS.

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @zooloo303 on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[zooloo303]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[zooloo303]

Updated the submission to v0.4.0 (was 0.2.0):

• moved the extensions.toml / .gitmodules entries to their alphabetical positions
• since the original submission the extension gained: a second language server providing live SAP backend syntax checks + ADT pull/push code actions, and an ABAP Icons icon theme (per-object-type file icons)

CLA was signed earlier in this PR. Ready for review 🙏