Fallback as a user-defined function for TypescriptGoToSourceDefinition

[dsych]

Allow users to supply the fallback function instead of assuming that they want to call the vanilla implementation.

For example, this would allow us to call telescope's implementation of go-to def:

require("typescript").goToSourceDefinition(winnr, {
    fallback = require("telescope.builtin").lsp_definitions
})

here we would simply check the type of the fallback and if it's a function, we would just call it.

[jose-elias-alvarez]

Sure, PR welcome.