naga-cli: Support reading input from stdin

[rhysd]

Current interface of naga-cli is as follows:

naga [OPTIONS] INPUT [OUTPUT...]

Since INPUT must be specified, the input file must exist in filesystem.

Since the input validation while editing shader program is great for those who are not familiar with WGSL like me. I'm considering to run naga command in a text editor on the fly.

However, while editing a source, the source text is not written to a file yet. It is only on memory of the text editor.

I suppose there are two possible solutions.

1. naga supports reading input from stdin (INPUT argument will be optional). A text editor can pass the current source via stdin
2. A text editor writes the current source to a temporary file and pass the path to naga

Cons of 1. is that it needs new implementation.
Cons of 2. is that it requires overhead of writing source to temporary file. The file path is different from actual file path being edited.

I think 1. is cleaner and faster.