Migrate to `{rlang}` 1.0.0

[Ilia-Kosenkov]

A lot of breaking changes in {rlang} release. For instance, all our error messages broke (see img attached). With recently added {cli} features we need to rework all of the ui_* (and related functions).

[JosiahParry]

There is one hiccup that needs to be addressed before this can be done completely. Namely, the function ui_throw() which throws an error adds a class rextendr_error to the error. The idea behind this is that it can be caught using tryCatch later down the line.

ui_throw() is ubiquitous. Replacing ui_throw() with cli::cli_abort() is identical except for the class.

The class is tested for in expect_rextendr_error. Removing ui_throw() necessitates that expect_error() replaces the rextendr specific version.

Do you feel comfortable deviating / altering the principles.md?

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Tasks if so:

• [x] replace ui_* functions with {cli} equivalents
• [x] modify or delete principles.md
• [x] replace expect_rextendr_error() with expect_error()
• [x] delete R/ui.R