Add Function.NewInstance to expose the corresponding V8 function

[dylanahsmith]

Problem

I was trying to do something like new Error(message) from Go code, but didn't see a way of using the javascript new operator.

Solution

It looks like v8::Function::NewInstance is the function we need for doing this, so I added a corresponding function to the v8go API.

[codecov]

Codecov Report

Merging #134 (dd357de) into master (b3de643) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   96.81%   96.90%   +0.08%     
==========================================
  Files          12       12              
  Lines         440      452      +12     
==========================================
+ Hits          426      438      +12     
  Misses          9        9              
  Partials        5        5              

Impacted Files	Coverage Δ
context.go	94.59% <100.00%> (+0.22%)	⬆️
function.go	100.00% <100.00%> (ø)

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[rogchap]

Hey @dylanahsmith
Thanks for this PR. Code looks good; would be good if we could raise the test coverage.
Could you also add an entry to the CHANGELOG.md?

[dylanahsmith]

I've added an extra test for the error case and a changelog entry. Looks like CI is awaiting approval to run