Simplify addHelpText TypeScript signature and improve typings test

[shadowspawn]

Pull Request

Problem

Declaring that .addHelpText() callback may return undefined makes for a messy signature. The intent was to allow a falsey value returned to skip display, by just calling return in callback.

Solution

Restrict to strings. User can easily return empty string to get the falsey behaviour of skipping display.

Improve the typings test of the callback.

ChangeLog

• TypeScript definition for .addTextHelp() callback no longer allows result of undefined, now just string.

[abetomo]

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