Implement Prism -> Sorbet translation for it keyword

[amomchilov]

Motivation

Closes #122.
Closes #229.

Test plan

See included automated tests.

[amomchilov]

The expected memory management pattern is:

pm_options_t options = {};
pm_string_t input = {};

pm_parser_t parser;
pm_parser_init(&parser, pm_string_source(&input), pm_string_length(&input), &options);

pm_node_t *node = pm_parse(&parser);
// do whatever

pm_node_destroy(&parser, node);
pm_parser_free(&parser);

pm_string_free(&input);
pm_options_free(&options);

In general a good place to look for this is extension.c (https://github.com/ruby/prism/blob/main/ext/prism/extension.c) in the Prism repo which effectively is a consumer of the C API of Prism for use in CRuby.

TODO: update this in the Prism C API docs.

[amomchilov]

ruby/prism#3066