Fix regex literals starting with equals

[davejcameron]

Problem

Regex literals whose pattern starts with = are valid in expression positions, but the parser rejects them before reading the regex body.

value.replace(/=/g, "")
value.replace(/=>/g, ":")

Those should parse and evaluate as regex literals, while compound assignment such as a /= b should still parse as assignment.

Fix

Allow = as the first regex pattern character when the / token is parsed in a prefix-expression position. The existing left-operand check still leaves division and /= assignment handling to their operator parsers.

Add focused regex feature coverage for replace(/=/g, ...) and replace(/=>/g, ...).

Validation

• node --input-type=module -e 'import { parse } from "./subscript.js"; import "./feature/regex.js"; console.log(JSON.stringify(parse("x.replace(/=/g, \\\"\\\")"))); console.log(JSON.stringify(parse("x.replace(/=>/g, \\\":\\\")"))); console.log(JSON.stringify(parse("a /= b")))'
• node --import ./test/https-loader.js test/feature/regex.js