get_source_expressions() should get a way to parse encodings directly

[MichaelChirico]

lintr/R/get_source_expressions.R

Line 64 in 700607e

source_expression <- srcfile(filename, encoding = settings$encoding)

Follow-up to #2803. Here as well we have get_source_expressions() relying on settings$encoding having been set elsewhere, meaning it's not very useful to call get_source_expressions() directly on a nonstandard-encoded file whose encoding can be inferred from settings.

Probably easiest to just expose encoding= as an argument, and/or possibly include parse_settings as an argument.

[AshesITR]

I would wait for the interface overhaul to parse_settings= (#1177) before exposing it. But that seems like a good call.

The new interface would allow specifying the encoding via custom settings anyway (e.g. config = list(encoding = "latin1"))