Support top level initializer block

[kamiazya]

In peggy@1.1.0, the function of top level initializer block is added, and constants and functions can be defined in the module.

Utilizing this feature in ts-pegjs solves the problem of type definition that was previously imported from the outside with customHeader, such as defining interface and type.

see peggyjs/peggy#73

Additional commemt

I was wondering where to insert the top level initializer block, but now I insert it at the top as peggy does.

[mikeatlas]

Came here looking for support for this. @pjmolina Any chance you might consider merging and releasing this PR?

[mikeatlas]

Note that if you're using the javascript.peg example, yes this will work to copy over the top-level initializer block, but my tsc really doesn't like the plain JS in strict mode - looks like the block in the example might be best if rewritten with typescript typed functions that get top-leveled.

[pjmolina]

Thank you @kamiazya & @mikeatlas
Looks good to me. Merging.

[mikeatlas]

Great! thanks @pjmolina @kamiazya 🙏