[SPARK-8537] [SPARKR] Add a validation rule about the curly braces in SparkR to `.lintr`

[[SPARK-8537] Add a validation rule about the curly braces in SparkR to `.lintr` - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-8537)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #6940 from yu-iskw/SPARK-8537 and squashes the following commits:

7eec1a0 [Yu ISHIKAWA] [SPARK-8537][SparkR] Add a validation rule about the curly braces in SparkR to `.lintr`

R/pkg/.lintr

-linters: with_defaults(line_length_linter(100), camel_case_linter = NULL)
+linters: with_defaults(line_length_linter(100), camel_case_linter = NULL, open_curly_linter(allow_single_line = TRUE), closed_curly_linter(allow_single_line = TRUE))
 exclusions: list("inst/profile/general.R" = 1, "inst/profile/shell.R")