fix(metadata-generator): Convert unsigned 64-bit enum values to signed

[mbektchiev]

This way most negative values will be correctly represented in JS and TS. Still values between 2^53 and 2^63 will have their representation converted to inexact doubles because the MAX_SAFE_INTEGER is 2^53-1

PR Checklist

• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [x] You have signed the CLA.
• [x] All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• [ ] Tests for the changes are included.

What is the current behavior?

.d.ts files define enum constants which have their 64th bit set as unsigned decimal integers

What is the new behavior?

.d.ts files define enum constants which have their 64th bit set as signed (negative) decimal integers to avoid misrepresenting them in JS due to the limitation that only integers up to 2^53-1(MAX_SAFE_INTEGER) can be represented exactly.

refs #1150