fix(tutorial): typing functions in tutorial

[yhsiang]

To fix #357

1. typing tutorial.js
2. typing handleInput.js
3. typing handlePostback.js

handlers/*.js should be next target

[coveralls]

Pull Request Test Coverage Report for Build 5864057925

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.005%) to 91.054%

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Files with Coverage Reduction	New Missed Lines	%
src/webhook/index.ts	2	50.0%

Totals
Change from base Build 5863858587:	0.005%
Covered Lines:	979
Relevant Lines:	1051

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💛 - Coveralls

[johnson-liang]

Nit: mind adding comment for the empty object (Record<string, never>)? It's not straightforward for why it is possible for data to be in this format.