[R-package] standardize style for placement of braces

[jameslamb]

In preparation for the release of {lintr} 3.0.0 (#5228), this PR proposes changes to the placement of braces in the project's R code.

Without these changes, {lintr} 3.0.0 will return the following findings:

LightGBM/R-package/R/lgb.convert_with_rules.R:7:33: style: Opening curly braces should never go on their own line and should always be followed by a new line.
            , FUN = function(x) {paste0(class(x), collapse = ",")}
                                ^
LightGBM/R-package/R/lgb.convert_with_rules.R:7:66: style: Closing curly-braces should always be on their own line, unless they are followed by an else.
            , FUN = function(x) {paste0(class(x), collapse = ",")}
                                                                 ^
LightGBM/R-package/R/lgb.convert_with_rules.R:43:51: style: Opening curly braces should never go on their own line and should always be followed by a new line.
.LGB_CONVERT_DEFAULT_FOR_LOGICAL_NA <- function() {return(-1L)}
                                                  ^
LightGBM/R-package/R/lgb.convert_with_rules.R:43:63: style: Closing curly-braces should always be on their own line, unless they are followed by an else.
.LGB_CONVERT_DEFAULT_FOR_LOGICAL_NA <- function() {return(-1L)}
                                                              ^
LightGBM/R-package/R/lgb.convert_with_rules.R:44:55: style: Opening curly braces should never go on their own line and should always be followed by a new line.
.LGB_CONVERT_DEFAULT_FOR_NON_LOGICAL_NA <- function() {return(0L)}
                                                      ^
LightGBM/R-package/R/lgb.convert_with_rules.R:44:66: style: Closing curly-braces should always be on their own line, unless they are followed by an else.
.LGB_CONVERT_DEFAULT_FOR_NON_LOGICAL_NA <- function() {return(0L)}
                                                                 ^
LightGBM/R-package/R/utils.R:72:58: style: Opening curly braces should never go on their own line and should always be followed by a new line.
    if (!all(sapply(interaction_constraints, function(x) {is.character(x) || is.numeric(x)}))) {
                                                         ^
LightGBM/R-package/tests/testthat/test_learning_to_rank.R:52:55: style: Opening curly braces should never go on their own line and should always be followed by a new line.
    expect_identical(sapply(eval_results, function(x) {x$name}), eval_names)
                                                      ^
LightGBM/R-package/tests/testthat/test_lgb.Booster.R:1206:20: style: Opening curly braces should never go on their own line and should always be followed by a new line.
    expect_warning({bst2 <- readRDS.lgb.Booster(file = model_file)})

[github-actions]

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