Dedupe completion options for commands

[SamBoyd]

Changes

This PR improves the command completion functionality by filtering out options that have already been mentioned in the current command.

For example, when using the /add command with multiple files, once a file has been typed, it won't be suggested again in the same command. This makes the completion suggestions more relevant and reduces clutter.

Another example might be when you're adding a module and its test module. After you've added the first file, you then have to figure out which is the module/test file again to prevent adding the same file twice. If the file name is quite large, it can be quite difficult to distinguish between the module and the test file.

It's not a large thing but happens often enough for it to be inconvenient.

Testing

Added a test case that verifies this behavior works correctly when completing file names in commands like /add file1.txt f where file2.txt should be suggested but not file1.txt again.