Enhance String.format function, support placeholder comment
Enhance String.format function, support placeholder comment

Usage: `"Hello, {0:userName}!".format("foobar")`
Output: `Hello, foobar!`
That is: the string between `:` and `}` will be just ignored.
Acceptable comment characters:
A-Z a-z 0-9 _ $ [more unicodes chars, such as CJK]
Invalid comment char will cause replacement fail.

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