Remove trailing _ from method and parameter names

In many cases, ReScript syntax is unambiguous enough that a trailing _ is no longer required in places it was for ReasonML and OCaml syntax. A codebase search for _: turns up quite a lot of them. Examples: https://github.com/tinymce/rescript-webapi/blob/ea9bd980f1b2b2d516e518f1f9c166feb1073f33/src/Webapi/Webapi__Blob.res#L10

https://github.com/tinymce/rescript-webapi/blob/253ba954381a78b57bad249ea83c1f5b6ab2b428/src/Webapi/Dom/Webapi__Dom__HtmlFormElement.res#L17

The compiler will signal when removing the underscore is not allowed, making this a mostly trivial change (but some tests would be nice)