Only return filenames when applicable in bash completion

[erichiller]

The current completion script for bash mixes filenames in with the dotnet commands and parameters creating a hard to find mess.

It would be nice if the completion script only returned files when they were appropriate, additionally the files should be listed after the parameters to the dotnet cli.

In the interim I modified the completion script to achieve a dumb version of this and it exhibits some of the desired properties:

• files only when valid (as best I could)
• parameters listed before files

Essentially it only returns the files if the only things returned from dotnet complete are prefixed with / or -. My assumption being that there will never be a file parameter without some command (eg. there's never a file directly after dotnet).

[baronfel]

Because completions are now managed entirely by System.CommandLine, I think we have a better way to describe this: for Arguments or Options that correspond to files, we should be able to write a completion function for GetCompletions that provides CompletionItems matching what that Argument or Option expects. For example dotnet sln <sln> add needs a PROJECT_PATH to apply, so we could suggest/complete only project files in the current directory (or the directory(ies) partially provided by the user). There's some discussion of this here if you'd like to add your thoughts, but we have some SDK work to do to define and annotate our Arguments/Options better.

[vijayrkn]

Removing the websdk tag since this is a general SDK item.

[baronfel]

Tagging @MiYanni just for visibility - the completions for the CLI all-up could be better, and this is one way in particular.