Introduce ScriptTrigger & CommandsTrigger for the R plugin

## Context

Part of #313. `plugin-scripts` wants every submodule to expose a `ScriptTrigger` and a `CommandsTrigger`, so a flow can be triggered whenever the underlying script/commands execution completes, with an **exit condition** deciding whether the trigger fires based on the result (e.g. only on failure).

`plugin-script-r` is missing both. Existing implementations in `plugin-script-go`, `plugin-script-node`, `plugin-script-python`, `plugin-script-ruby`, and `plugin-script-shell` (same package as the task classes, no nested subpackage) can be used as a reference.

## What to implement

### 1. `ScriptTrigger`

- Triggers when `io.kestra.plugin.scripts.r.Script` execution completes.
- Emits an event containing: command executed, arguments, env (if available), exit code, stdout/stderr (if capture enabled), and contextual metadata (execution id, namespace, flow id).

### 2. `CommandsTrigger`

- Triggers when `io.kestra.plugin.scripts.r.Commands` execution completes.
- Emits an event containing: commands executed (or the matched one), exit code(s), stdout/stderr if available, and contextual metadata (execution id, namespace, flow id).

## Trigger configuration requirements

Each trigger should accept at least:

- `commandPattern` (string / regex / glob)
- `argumentsPattern` (optional)
- `exitCondition` (string expression — can match on exit code, output, or a literal condition)

## Example YAML

```yaml
triggers:
- id: on_r_fail
type: io.kestra.plugin.scripts.r.ScriptTrigger
commandPattern: ".*"
exitCondition: "exitCode != 0"

triggers:
- id: on_r_commands
type: io.kestra.plugin.scripts.r.CommandsTrigger
commandPattern: ".*"
exitCondition: "exitCode == 0"
```

Compare the existing ScriptTrigger and CommandsTrigger implementations in plugin-script-go, plugin-script-node, plugin-script-python, plugin-script-ruby, and plugin-script-shell, then inspect the R Script and Commands task classes. Follow how those implementations build events and evaluate patterns and exit conditions. Done means both R triggers emit the requested execution context and output fields while honoring command, argument, and exit-condition configuration.

Written by the indexing model from the issue text.