Add matching brackets exercise

[kahgoh]

(Also updated config.json to add references to exercise files)

This is one of the 48in24 exercises.

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[keiravillekode]

lgtm

[keiravillekode]

Let's make this difficulty 5 or 6.

[keiravillekode]

Let's add an instructions.append.md

My suggestion would be to copy exercises/practice/rotational-cipher/.docs/instructions.append.md which gives the student permission to modify the input buffer.

[kahgoh]

Thanks for the review @keiravillekode! I have pushed the commits for the difficulty and the adding the instructions.append.md. I can't see it here on the PR yet, but I assume they'll become visible when re-opened.

[keiravillekode]

We can simply have

(call $initBracketStack (local.get $text))

Once we have read the first characters of $text, their memory can be reused for the bracket stack.

With the current tests it does not matter, but it would be important if we had a test where the input was more than 127 bytes long, and was all opening brackets.