feat: add queries for R

[wurli]

Adds support for R. Only adds one query for function definitions, but this is really all that's needed since R is a highly functional language.

I also added a test file r_test.R, but I wasn't able to get tests to run properly. Is there more to this than sourcing tests/minimal_init.lua and then using :RunTests?

Thanks!

[stevearc]

You're going to want to run make update_snapshots to generate the symbols snapshot for the test, after which make test will run the tests

[wurli]

You're going to want to run make update_snapshots to generate the symbols snapshot for the test, after which make test will run the tests

Thanks! symbols/r_test.json now added :)

[stevearc]

Looks good, thanks for the PR!