[mathiasrw]

Closes #815

Stacked on #830.

Change

The numbers of #791, the groups and the named groups of #799 are made with var in front of the --js code. Code that declared one of those names itself with let or const did not run:

> echo -n foo | rexreplace o 'let $t = "T"; $t' --js
 ❌ The code of --js failed for the piped data: SyntaxError: Cannot declare a let variable twice: '$t'.

Now the code of the user is put in a block of its own, so its let and const are in their own scope, and it gives fTT again. The value of a block is the value of its last statement, so the code works as before. A line break goes before the end of the block, so a // comment at the end of the code does not hide it.

Tests

test/fix/815.test.js is new, and its tests with let and const fail without the fix. yarn test-ci passes.

From the review of 23 October.

Follow-up

test/fix/815.test.js also covers a cousin with const ($n00), a function named like a number, and that var $t still works. The var and comment tests pass without the fix too, as guards.