feat: support vscode tasks for exporting pdf, svg, and png (#488)

* feat: support vscode tasks for exporting pdf, svg, and png

* fix: parse errors

* dev: update fill, gap arguments

* fix: merged props

docs/tinymist/book.typ

@@ -14,6 +14,7 @@
     = Features
     - #chapter("feature/cli.typ")[Command line interface]
     - #chapter("feature/language.typ")[Language and Editor Features]
+    - #chapter("feature/export.typ")[Exporting Documents]
     - #chapter("feature/preview.typ")[Document Preview]
     = Guide
     - #chapter("guide/completion.typ")[Completion]

docs/tinymist/feature/export.typ

@@ -0,0 +1,70 @@
+#import "mod.typ": *
+
+#show: book-page.with(title: "Exporting Documents")
+
+You can export your documents to various formats using the `export` feature.
+
+== VSCode: Task Configuration
+
+You can configure tasks in your `tasks.json` file to "persist" the arguments for exporting documents.
+
+Example:
+
+```json
+{
+  "version": "2.0.0",
+  "tasks": [
+    {
+      "label": "Export SVG",
+      "type": "typst",
+      "command": "export",
+      "export": {
+        "format": "svg",
+        "merged": true
+      }
+    },
+    {
+      "label": "Export PNG",
+      "type": "typst",
+      "command": "export",
+      "export": {
+        "format": "png",
+        // Default fill is white, but you can set it to transparent.
+        "fill": "#00000000",
+        "merged": true
+      }
+    },
+    {
+      "label": "Export PNG and SVG",
+      "type": "typst",
+      "command": "export",
+      "export": {
+        "format": ["png", "svg"],
+        // To make a visual effect, we set an obvious low resolution.
+        // For a nice result, you should set a higher resolution like 288.
+        "png.ppi": 24,
+        "merged": true,
+        // To make a visual effect, we set an obvious huge gap.
+        // For a nice result, you should set a smaller gap like 10pt.
+        "merged.gap": "100pt"
+      }
+    }
+  ]
+}
+```
+
+todo: documenting export options.
+
+After configuring the tasks, you can run them using the command palette.
++ Press `Ctrl+Shift+P` to open the command palette.
++ Type `Run Task` and select the task you want to run.
++ Select the task you want to run.
+
+== Neovim: Export Commands
+
+You can call the following export commands.
+- `tinymist.exportSvg`
+- `tinymist.exportPng`
+- `tinymist.exportPdf`
+
+The first argument is the path to the file you want to export and the second argument is an object containing additional options.