[TS] Fix FileHeader input type

[Shimmi]

Issue #, if available: N/A

Description of changes:

Change the FileHeader's defaultMessage input type string[] instead of just string.

Reason:

File Header's defaultMessage is defined as an array in code.

File Header in Examples shows expanding the defaultMessage. E.g. return [...defaultMessage, 'hello world']. TypeScript won't allow this currently due to the string data type.

Altho the current data type works, it implicitly converts the array into a string which results in a malformed header.

// Do not edit directly,Generated on Sat, 02 Oct 2021 12:57:05 GMT
// hello, world!

vs (fixed)

// Do not edit directly
// Generated on Sat, 02 Oct 2021 12:45:13 GMT
// hello, world!

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P.S. Is there a way to change the header for every file without the need to specify the custom header formater in every file/platform? Currently, I do following:

Object.keys(config.platforms).forEach((key) => {
    config.platforms[key].options = {fileHeader: 'myHeader', ...config.platforms[key].options}
})

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dbanksdesign]

@Shimmi currently there are no global-level options like fileHeader that cascade down to platforms and then to files. As you point out, you can define a fileHeader at the platform-level and it will get applied to all files in that platform.