Only start of index expansion needs to be quoted for quoted variable expansion

Running fish version 3.2.2 (installed via Homebrew) on macOS 11.3 in the stock Terminal.app. I tried fish without third-party customisations by executing `sh -c 'env HOME=$(mktemp -d) fish'` and checked whether it affected the behaviour I am reporting (it did not).

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_Einen habe ich noch_\*

The following is related to, but distinct from, #7969 and, AFAICS, not covered by an open issue yet (apologies if I missed something in the depths of #2485).

I’ve noticed (while, mumble, Nova syntax extension, mumble; yes I am really, truly, frightfully sorry) that, **for a double quoted variable expansion, index expansion needs not be quoted in its entirety**. If the opening bracket is included in the quotes, that is enough; see the examples below:

```fish
set var (seq 2)
echo "$var[1..2"] # => 1 2
echo "$var[1.."2] # => 1 2
# index range operator cannot be split up …
echo "$var[1"..2] # => 1 2
echo "$var["1..2] # => 1 2
echo "$var"[1..2] # => 1 2[1..2]
```

This seems counterintuitive, as I am not aware of any other pair-delimited expression that keeps its signification across a quote; at odds with fish’s handling of variable expansion otherwise (the `$` operator(s) and identifier characters cannot cross a quote; neither can the indexing follow after a quote in its entirety); and internally inconsistent, as you cannot do it the other way around (i.e. `echo $var[1.."2]"` throws a syntax error about unmatched square brackets).

\* One more thing.