fix: Fix modules export with inject fn

[tjenkinson]

We noticed if you use use the 'inject' mode with a function the module exports object is not included.

This fixes that.

Also it changes const to var so that the output is es5. I think this makes sense given even though rollup itself supports es6, it keeps anything it generates itself to es5.

E.g.

export var test = 1;

becomes

var myBundle = (function (exports) { // <--- var
    'use strict';

    var test = 1;

    exports.test = test;

    return exports;

}({}));

with the iife format.

The typescript compiler was also also complaining because it looks like rollup added support for assetFileNames to be a function, which is currently not handled. I updated the code to throw an exception in this case for now.

[codecov[bot]]

Codecov Report

Merging #128 into master will decrease coverage by 0.09%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   99.60%   99.51%   -0.10%     
==========================================
  Files          36       36              
  Lines        1020     1022       +2     
  Branches      266      267       +1     
==========================================
+ Hits         1016     1017       +1     
- Misses          1        2       +1     
  Partials        3        3              

Impacted Files	Coverage Δ
src/index.ts	98.78% <50.00%> (-0.61%)	:arrow_down:
src/loaders/postcss/index.ts	100.00% <100.00%> (ø)

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[Anidetrix]

Thank you for your contribution @tjenkinson! Also thank you for noticing assetFileNames change, was busy lately and missed that, I'll fix that as soon as I can.

[tjenkinson]

Thanks for the super quick review! There was another place where const is used. I'll open a new PR