Rmd chunk symbol

[renkun-ken]

This PR enhances #186 by adding chunk symbols in addition to section symbols for R markdown documents.

Also requested at REditorSupport/vscode-R#363.

For the following document,

---
title: new document
---

# hello

Hello, world!

```{r}
x <- rnorm(100)
y <- rnorm(100)
m <- lm(y ~ x)

fun1 <- function(x, y) {
  x + y
}
```

```{r chunk-1}
x <- rnorm(100)
y <- rnorm(100)
m <- lm(y ~ x)

fun2 <- function(x, y) {
  x + y
}
```

```{r chunk-2, eval=TRUE}
x <- rnorm(100)
y <- rnorm(100)
m <- lm(y ~ x)

fun3 <- function(x, y) {
  x + y
}
```

```{r "chunk-3"}
x <- rnorm(100)
y <- rnorm(100)
m <- lm(y ~ x)

fun4 <- function(x, y) {
  x + y
}
```

```{r 'chunk-4', eval=TRUE}
x <- rnorm(100)
```

```{r 'chunk-5, new test', eval=TRUE}
x <- rnorm(100)
```

```{r}
x <- 1
```

we could get the following symbols:

[randy3k]

LGTM. Thanks.