This implements support for named captures in
RegExp.prototype[@@replace] for when the replaceValue is not callable.

Named captures can be referenced from replacement strings by using the
"$<name>" syntax. A couple of examples:

let re = /(?<fst>.)(?<snd>.)/u;
"abcd".replace(re, "$<snd>$<fst>")  // "bacd"
"abcd".replace(re, "$2$1")     // "bacd" (numbered refs work as always)
"abcd".replace(re, "$<snd")    // SyntaxError (unterminated named ref)
"abcd".replace(re, "$<42$1>")  // "cd" (invalid name)
"abcd".replace(re, "$<thd>")   // "cd" (non-existent name)
"abcd".replace(/(?<fst>.)|(?<snd>.)/u, "$<snd>")  // "cd" (non-matched capture)

Support is currently behind the --harmony-regexp-named-captures flag.

BUG=v8:5437

Review-Url: https://codereview.chromium.org/2775303002
Cr-Commit-Position: refs/heads/master@{#44171}