Migration of jsx files to tsx 2 (#850)

This is to continue the work of migrating all the jsx files into tsx
files.
The MO is basically described here: #848.

About 7 files to go...

src/components/FieldComment.tsx

@@ -0,0 +1,30 @@
+import React from 'react'
+
+import Block from './Block'
+import InputString from './InputString'
+
+type FieldCommentProps = {
+  value?: string
+  onChange(...args: unknown[]): unknown
+};
+
+export default class FieldComment extends React.Component<FieldCommentProps> {
+  render() {
+    const fieldSpec = {
+      doc: "Comments for the current layer. This is non-standard and not in the spec."
+    };
+
+    return <Block
+      label={"Comments"}
+      fieldSpec={fieldSpec}
+      data-wd-key="layer-comment"
+    >
+      <InputString
+        multi={true}
+        value={this.props.value}
+        onChange={this.props.onChange}
+        default="Comment..."
+      />
+    </Block>
+  }
+}