Update JSXSlackTemplateTag type: Accept readonly string array as template

[yhatt]

TypeScript built-in TemplateStringsArray type (used by htm) requires having raw propoerty.

interface TemplateStringsArray extends ReadonlyArray<string> {
    readonly raw: readonly string[];
}

raw is not supported by jsx-slack, so I updated JSXSlackTemplateTag type to accept just a readonly string array as template.

This change prevents type error in the (rare) case of directly use jsxslack as a function, just like jsxslack([...], ...).

Related: #311