## What changes were proposed in this pull request?

Currently, multi-dot separated variables in R is not allowed. For example,

```diff
 setMethod("from_json", signature(x = "Column", schema = "structType"),
-          function(x, schema, asJsonArray = FALSE, ...) {
+          function(x, schema, as.json.array = FALSE, ...) {
             if (asJsonArray) {
               jschema <- callJStatic("org.apache.spark.sql.types.DataTypes",
                                      "createArrayType",
```

produces an error as below:

```
R/functions.R:2462:31: style: Words within variable and function names should be separated by '_' rather than '.'.
          function(x, schema, as.json.array = FALSE, ...) {
                              ^~~~~~~~~~~~~
```

This seems against https://google.github.io/styleguide/Rguide.xml#identifiers which says

> The preferred form for variable names is all lower case letters and words separated with dots

This looks because lintr by default https://github.com/jimhester/lintr follows http://r-pkgs.had.co.nz/style.html as written in the README.md. Few cases seems not following Google's one as "a few tweaks".

Per [SPARK-6813](https://issues.apache.org/jira/browse/SPARK-6813), we follow Google's R Style Guide with few exceptions https://google.github.io/styleguide/Rguide.xml. This is also merged into Spark's website - https://github.com/apache/spark-website/pull/43

Also, it looks we have no limit on function name. This rule also looks affecting to the name of functions as written in the README.md.

> `multiple_dots_linter`: check that function and variable names are separated by _ rather than ..

## How was this patch tested?

Manually tested `./dev/lint-r`with the manual change below in `R/functions.R`:

```diff
 setMethod("from_json", signature(x = "Column", schema = "structType"),
-          function(x, schema, asJsonArray = FALSE, ...) {
+          function(x, schema, as.json.array = FALSE, ...) {
             if (asJsonArray) {
               jschema <- callJStatic("org.apache.spark.sql.types.DataTypes",
                                      "createArrayType",
```

**Before**

```R
R/functions.R:2462:31: style: Words within variable and function names should be separated by '_' rather than '.'.
          function(x, schema, as.json.array = FALSE, ...) {
                              ^~~~~~~~~~~~~
```

**After**

```
lintr checks passed.
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17590 from HyukjinKwon/disable-dot-in-name.