[ci] [R-package] enforce more {lintr} checks

[jameslamb]

Proposes the following:

• adds more linters from {lintr} to this project's R linting
• alphabetizes the list of linters in the R linting script
• adds the base R pipe (|>) to the list of operators not allowed to be used in this project's R code

Fixes the following new errors from {lintr}:

[[1]]
/.ci/lint_r_code.R:2:1: warning: [unused_import] Package 'lintr' is only used by namespace. Check that it is installed using loadNamespace() instead.
library(lintr)
^~~~~~~~~~~~~~

[[2]]
/R-package/demo/basic_walkthrough.R:2:1: warning: [unused_import] Package 'methods' is attached but never used.
library(methods)
^~~~~~~~~~~~~~~~

[[3]]
/R-package/demo/boost_from_prediction.R:2:1: warning: [unused_import] Package 'methods' is attached but never used.
library(methods)
^~~~~~~~~~~~~~~~

[[4]]
/R-package/demo/early_stopping.R:2:1: warning: [unused_import] Package 'methods' is attached but never used.
library(methods)
^~~~~~~~~~~~~~~~

[[5]]
/R-package/tests/testthat.R:2:1: warning: [unused_import] Package 'lightgbm' is attached but never used.
library(lightgbm)
^~~~~~~~~~~~~~~~~

[[6]]
/R-package/tests/testthat/test_dataset.R:186:33: warning: [implicit_assignment] Avoid implicit assignments in function calls. For example, instead of `if (x <- 1L) { ... }`, write `x <- 1L; if (x) { ... }`.
  expect_silent(colnames(dtest) <- new_names)
                                ^~

[[7]]
/R-package/tests/testthat/test_learning_to_rank.R:40:9: warning: [conjunct_test] Instead of expect_true(A && B), write multiple expectations like expect_true(A) and expect_true(B) The latter will produce better error messages in the case of failure.
        expect_true(result[["value"]] > 0.0 && result[["value"]] < 1.0)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[[8]]
/R-package/tests/testthat/test_learning_to_rank.R:107:5: warning: [conjunct_test] Instead of expect_true(A && B), write multiple expectations like expect_true(A) and expect_true(B) The latter will produce better error messages in the case of failure.
    expect_true(best_iter > 0L && best_iter <= nrounds)
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[[9]]
/R-package/tests/testthat/test_learning_to_rank.R:108:5: warning: [conjunct_test] Instead of expect_true(A && B), write multiple expectations like expect_true(A) and expect_true(B) The latter will produce better error messages in the case of failure.
    expect_true(best_score > 0.0 && best_score < 1.0)
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[[10]]
/R-package/tests/testthat/test_lgb.Booster.R:1278:34: warning: [implicit_assignment] Avoid implicit assignments in function calls. For example, instead of `if (x <- 1L) { ... }`, write `x <- 1L; if (x) { ... }`.
    expect_warning(bst_from_file <- readRDS.lgb.Booster(file = bst_file))
                                 ^~

Notes for Reviewers

See https://lintr.r-lib.org/reference/index.html for docs on these new linters.

[jameslamb]

Ah interesting, having unused_import_linter() throws a bunch of errors on packages that aren't installed: https://github.com/microsoft/LightGBM/actions/runs/6438788536/job/17485700424?pr=6130

Removed that linter in cf40d50, to avoid needing to install all those packages in the lint CI job.

Ah it was actually namespace_linter! My mistake. Removed in f004947.

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