Disallow template substitution in the path of the URL template

[mgiuca]

This prevents a path escape if the share data includes "..". While it is a bit restrictive, we expect templates to mostly be in the query or fragment part of a URL.

Closes #30.

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[mgiuca]

@marcoscaceres and @ericwilligers PTAL.

There's a follow-up to this in mgiuca/web-share-target/parse-at-parse-time, which changes the processing and validation to take place at manifest load time. But I separated this change out.

[marcoscaceres]

Looks good. I guess the only thing would be to allow "replace placeholders" to do the null check, and just return an empty string. That would remove a few ifs, but no big deal.

[mgiuca]

I guess the only thing would be to allow "replace placeholders" to do the null check, and just return an empty string. That would remove a few ifs, but no big deal.

I thought about this a bit, and decided not to. Rationale: This would make the "replace placeholders" algorithm take a maybe-null value and return a maybe-null value. It would return null if-and-only-if the input is null. I generally prefer that functions do not take and return null values, and if the only reason to return null is in case the input is null, I prefer that the caller simply check beforehand. (As a general programming principle.)