[R-package] Rename slice() to lgb.slice.Dataset()

[david-cortes]

From similar PR in xgboost: dmlc/xgboost#10017

LightGBM registers a generic S3 slice method, and defines it for its class lgb.Dataset.

This is problematic in R, since a widely used library, {dplyr}, also defines a generic slice method but with a different signature, with implementations for data frames and variants, and extensions like {dbplyr} and similar define it for more object classes based on dplyr's signature.

This PR renames it to lgb.slice.Dataset, in order to avoid conflicts with other commonly used R frameworks.

[jameslamb]

Thanks for this!
I'm ok with making this breaking change, given that:

• it's already been merged over in XGBoost (that PR you linked)
• {dplyr} is probably much more widely used than {lightgbm}
• the use of :: in some R isn't as common as I wish it was 😞
• as far as I can tell from a quick code search, {lightgbm}'s reverse dependencies on CRAN don't use slice()

For others wondering what specifically "problematic" in this PR description refers to, it's this:

library(lightgbm)
library(dplyr)

# Attaching package: ‘dplyr’
# The following object is masked from ‘package:lightgbm’:
#
#    slice

dtrain <- lgb.Dataset(
    data = matrix(rnorm(1e5), nrow = 1000)
)
dtrain$construct()

slice(dtrain, 1:5)
# Error in UseMethod("slice") :
#  no applicable method for 'slice' applied to an object of class "c('lgb.Dataset', 'R6')"

If slice() is called on a Dataset in a session where {dplyr} was attached after {lightgbm}, the {lightgbm} method isn't found and a runtime error occurs. This can be avoided by such code specifying lightgbm::slice().

[jameslamb]

This error in CI is unrelated to these changes:

Run r-lib/actions/setup-pandoc@v2
  with:
    pandoc-version: [2](https://github.com/microsoft/LightGBM/actions/runs/7745434225/job/21139413173?pr=6293#step:7:2).19.2
  env:
    _R_CHECK_SYSTEM_CLOCK_: 0
    _R_CHECK_CRAN_INCOMING_REMOTE_: 0
    _R_CHECK_PKG_SIZES_THRESHOLD_: 100
/__e/node20/bin/node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /__e/node20/bin/node)

From the combination of actions/checkout#1474 and https://github.com/r-lib/actions/releases, I suspect the root cause is r-lib/actions/setup-pandoc updating to a newer version of node. I'll work around that on a separate PR.