tools: allow single JS file for --link-module

The description for the --link-module configuration option is as follows:

$ ./configure --help | grep -A 5 'link-module'
  --link-module=LINKED_MODULE
                      Path to a JS file to be bundled in the binary as a
                      builtin. This module will be referenced by path
                      without extension; e.g. /root/x/y.js will be
                      referenced via require('root/x/y'). Can be used
                      multiple times

This lead me to think that it was possible to specify a file like this:

$ ./configure --link-module=something.js
$ NODE_DEBUG=mkcodecache make -j8

This will lead to a compilation error as an entry in the source_ map in node_javascript.cc will end up having an empty string as its key:

source_.emplace("", UnionBytes{_raw, 105});

This will then be used by CodeCacheBuilder when it iterates over the module ids, which will lead to the following compilation errors:

/node/out/Release/obj/gen/node_code_cache.cc:12:23: warning:
ISO C++17 does not allow a decomposition group to be
empty [-Wempty-decomposition]
static const uint8_t [] = {
                      ^
/node/out/Release/obj/gen/node_code_cache.cc:12:22: warning:
decomposition declarations are a C++17 extension [-Wc++17-extensions]
static const uint8_t [] = {
                     ^~
/node/out/Release/obj/gen/node_code_cache.cc:12:1: error:
decomposition declaration cannot be declared 'static'
static const uint8_t [] = {
^~~~~~
/node/out/Release/obj/gen/node_code_cache.cc:12:22: error:
decomposition declaration cannot be declared with type 'const uint8_t'
(aka 'const unsigned char'); declared type must be 'auto' or
reference to 'auto'
static const uint8_t [] = {
                     ^
/node/out/Release/obj/gen/node_code_cache.cc:12:22: error:
excess elements in scalar initializer
static const uint8_t [] = {
                     ^
/node/out/Release/obj/gen/node_code_cache.cc:660:7: error:
expected expression
      ,
      ^
/node/out/Release/obj/gen/node_code_cache.cc:661:24: error:
no matching function for call to 'arraysize'
      static_cast<int>(arraysize()), policy
                       ^~~~~~~~~
../src/util.h:667:18: note: candidate function template not viable:
requires 1 argument, but 0 were provided
constexpr size_t arraysize(const T (&)[N]) {
                 ^
2 warnings and 5 errors generated.

This commit suggests that passing a single file be allowed by modifying tools/js2c.py, or alternatively raise an Exception with an message that indicates the cause of this.

With the changes in this PR the usage would look like this:

$ cat something.js
'use strict';

function logit() {
  console.log('something module...');
}

module.exports = {
  logit,
}
$ ./configure --link-module=something.js && NODE_DEBUG=mkcodecache make -j8
...
Generated cache for something, size = 840.00B, total = 643.00KB
...
$ rm something.js
$ ./node -p "require('something').logit()"
something module...
undefined

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines