Fix TextMate Scope for '.ice' Files

[InsertCreativityHere]

Hello! I'm the maintainer of the Slice language!

A few months ago, we released a new 'edition' of our language's syntax, so there are now 2 separate syntaxes for Slice.
By convention, the original syntax uses (and has always used) .ice file suffixes. The new syntax uses .slice file suffixes.
Due to the difference in syntax, each uses it's own highlighter. You can see them here:
https://github.com/zeroc-ice/vscode-slice/tree/main/syntaxes
Note that this is already the grammar repo used by Linguist:

linguist/vendor/README.md

Line 517 in fd10bee

- **Slice:** [zeroc-ice/vscode-slice](https://github.com/zeroc-ice/vscode-slice)

As part of this change, we modified the TM scope associated with .ice files from source.slice to source.ice
(so we could use the source.slice TM scope for the new syntax).
This change took place here: zeroc-ice/vscode-slice@ab6bf2d

I acknowledge this is probably a fairly unorthodox thing to do, so any guidance on what's the best way to proceed would be appreciated! But, as the absolute least, I opened this PR because the highlighting for .ice files is currently broken, as they're pulling in the TM scope for the new syntax.

Checklist:

• I am changing the source of a syntax highlighting grammar
  • The grammar repo is still https://github.com/zeroc-ice/vscode-slice/blob/main/syntaxes/ice.tmLanguage.json
    This isn't changed by this PR.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.