Rework --generator Command Line Format

[InsertCreativityHere]

This PR implements the changes outlined in #747 (comment).
If anyone has any other changes to request with the generator CLI, feel free to comment on that issue!

Otherwise, as-of this PR, I consider #747 as completed.

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For the tooling, they will no longer need to replace \\ with /, but they still have to replace , with \, in the generator path.
This is sufficient for any generator to be parsed correctly by slicec.
Generators do not need to escape =. We don't ascribe any special meaning to = in the path, only in the arguments.

[Copilot]

Pull request overview

This PR updates slicec’s --generator command-line format to match the new proposal from issue #747, switching generator argument separation to a comma-based format and adjusting escaping rules accordingly.

Changes:

• Reworked generator CLI parsing to use , as the path/argument separator and = within arguments, with \ used to escape , and = when needed.
• Updated command-line parsing tests to cover trailing separators and the revised escaping behavior.
• Adjusted validation/error messaging around missing keys/paths to align with the new format.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
slicec/src/slice_options.rs	Updates the generator plugin parser to the new comma-separated format and adds validation for empty path / empty keys.
slicec/tests/command_line_tests.rs	Updates and expands CLI parsing tests to reflect the new --generator syntax and escaping semantics.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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