move stdio to dedicated package & fix directed stdio handling

[Guest0x0]

Previously, stdin/stdout/stderr is placed in @pipe, and assumed to be a pipe. But this is incorrect, because stdio may be redirected to any kind of fd, such as regular file. In this case, the current @pipe.stdxxx is simply broken.

This PR moves stdin/stdout/stderr to a dedicated package moonbitlang/async/stdio, with proper handling of any file kind. The previous @pipe.stdxxx are still kept for compatibility purpose, but deprecated.

[coveralls]

Pull Request Test Coverage Report for Build 658

Details

• 10 of 36 (27.78%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 78.543%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/fs/fs.mbt	2	3	66.67%
src/process/process.mbt	0	2	0.0%
src/pipe/pipe.mbt	0	3	0.0%
src/stdio/stdio.mbt	0	20	0.0%

Totals
Change from base Build 657:	-0.2%
Covered Lines:	1164
Relevant Lines:	1482

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💛 - Coveralls