Allow quotes to be escaped in JSON path

[lnkuiper]

We conveniently support escaping JSON object keys that contain the special [ and . characters by surrounding them with quotes.

{
  "duck": [42, 43]
}

The path $.duck points to [42, 43], and the path $.duck[0] points to 42.

If the key was "du[ck" instead, we wouldn't be able to extract because we would interpret [ as the syntax for extracting from an array. We already implemented escaping these special characters by surrounding them with quotes: $."du[ck".

If the key was "du\"ck" instead, we would also be able to extract, by just specifying the path $.du"ck. However, if it is necessary to escape special characters like [, but keys also contain quotes, you were out of luck.

So, we would not be able to extract if the key was, for example, "du[\"ck". Surrounding the path with quotes does not work because there is a quote in the middle of the key.

This PR solves this problem by allowing quotes within quoted paths to be escaped with a backslash:

select '{"d[u]_\"ck":42}'->'$."d[u]_\"ck"' v;
┌──────┐
│  v   │
│ json │
├──────┤
│ 42   │
└──────┘

Fixes #11997

[Mytherin]

Thanks for the PR! Looks good - some comments:

[Mytherin]

Should this not be an error if backslash is true?

[lnkuiper]

we ignore other backslashes as we do for regular (non quoted) JSON paths

[Mytherin]

Hm, but then returning an empty result here doesn't seem correct to me - shouldn't we match x\ as the string literal 'x\' in that case, instead of returning an empty result? Could we add such a test?

[lnkuiper]

Ah right, I understand the confusion. If ptr == end || backslash, we return an empty result, which will cause a descriptive error, pointing to the location where the error occurred. So, if we have $."x\ as a path, you will get an error

[Mytherin]

That seems a bit inconsistent, no? We accept a backslash as a regular backslash except when it is the last character? I would say we should turn it into a regular backslash in this scenario perhaps?

[lnkuiper]

When you have the path $."x\, we need to throw an error, because the path is "escaped", i.e., the object key starts with a ", so it needs to end with a " too. If it ends with a backslash, it's wrong. Actually, I just realised that it can be removed from the check, since, ptr == end will be true

[Mytherin]

Thanks!