Extend regular expression fuzzer dictionary

We’ve recently shipped some new regular expression features in V8.
This patch extends the fuzzer dictionary with the new syntax for:

- positive lookbehind assertions
- negative lookbehind assertions
- named capture groups
- Unicode property escapes

More information on these features can be found here:

- https://mathiasbynens.be/notes/es-regexp-proposals
- https://developers.google.com/web/updates/2017/07/upcoming-regexp-features

BUG=v8:7225

Change-Id: Ia506ba2282932fce43b02f1eccf327c8cd869101
Reviewed-on: https://chromium-review.googlesource.com/832526
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524714}

testing/libfuzzer/fuzzers/dicts/regexp.dict

@@ -219,3 +219,21 @@
 "\x0f"
 "[-\xf0\x9f\x92\xa9]+"
 "[\xf0\x9f\x92\xa9-\xf4\x8f\xbf\xbf]"
+"(?<=)"
+"(?<=a)"
+"(?<!)"
+"(?<!a)"
+"(?<a>)"
+"(?<a>.)"
+"(?<a>.)\\k<a>"
+"\\p{Script=Greek}"
+"\\P{sc=Greek}"
+"\\p{Script_Extensions=Greek}"
+"\\P{scx=Greek}"
+"\\p{General_Category=Decimal_Number}"
+"\\P{gc=Decimal_Number}"
+"\\p{gc=Nd}"
+"\\P{Decimal_Number}"
+"\\p{Nd}"
+"\\P{Any}"
+"\\p{Changes_When_NFKC_Casefolded}"