Deprecate asm stmt for js target

[ASVIEST]

why ?

• We already have an emit that does the same thing
• The name asm itself is a bit confusing, you might think it's an alias for asm.js or something else.
• The asm keyword is used differently on different compiler targets (it makes it inexpressive).
• Does anyone (other than some compiler libraries) use asm instead of emit ? If yes, it's a bit strange to use asm somewhere and emit somewhere. By making the asm keyword for js target deprecated, there would be even less use of the asm keyword for js target, reducing the amount of confusion.
• New users might accidentally use a non-universal approach via the asm keyword instead of emit, and then when they learn about asm, try to figure out what the differences are.

see https://forum.nim-lang.org/t/10821

[Araq]

Tests are red btw.

[ASVIEST]

The standard library had asm instead of emit, and the tester reacted to DeprecationWarning, now it should pass the tests.

[github-actions]

Thanks for your hard work on this PR!
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