Define how to extract the sourceMappingURL comment

[nicolo-ribaudo]

This PR follows the discussion from last week about tc39/source-map#64. It's marked as a draft because I still haven't "translated" the JS part to CSS, but please already check the JS part and express your opinion :)

Rendered version: https://nicolo-ribaudo.github.io/source-map-spec/source-map.html#linking-generated-code

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This patch explicitly defines how to extract such comments from JavaScript, CSS and WebAssembly sources.

It defines multiple ways to do so: either by actually parsing the code, or by just going through all the lines of the program looking for what "looks like" a comment. This is so that different implementations can choose what's best for them, depending on whether they are already parsing the code or not.

To ensure consist behavior accross implementations that choose different strategies, the specification enforces additional requirements on tools that append a sourceMappingURL comment to the generated code: the comment must be placed in such a way that all extraction methods yield the same result. This is not an unresonable burden, since if the progeram is syntactically valid, simply adding the comment at the end of the file only potentially followed by other tool-injected comments is enough. This requirement is lifted if the input code given to the tool is already "maliciously crafted", since we would otherwise require tool to go rewrite that code (for example, splitting strings that contain something that looks like a comment).

I have left the CSS extraction method as TODO because first I want to check how do you feel about the JS one. It has the following properties:

• It iterates line by line. Implementations can thus optimize it by going through each line in reverse order, and then scanning through its characters from the beginning to the end (which is what a regexp would do).
• It expects multi-line comments to actually be in a single line.
• It returns the last sourceMappingURL comment (or well, comment-like) found in the source.
• It only considers comments after the last piece of code (i.e. it discards any comment found so far every time it sees some non-comment non-whitespace characters).
• It has no requirements about what is before a comment. Adding the comment at the end of the file without first ensuring that there is a newline before it is valid.

[gibson042]

Is it a problem that ECMAScript white space is subject to change over time as future Unicode editions change the set of code points in general category "Space_Separator"?