textDocumentPositionParams: add optional range

This change adds an optional range field to textDocumentPositionParams
so that clients can, for example, query a subexpression such as
x.f within a larger expression g(x.f[i]).

Also, clarify the text about cursor bias.

Fixes #377
Fixes #1029

_specifications/lsp/3.18/types/textDocumentPositionParams.md

@@ -2,7 +2,18 @@
 
 Was `TextDocumentPosition` in 1.0 with inlined parameters.
 
-A parameter literal used in requests to pass a text document and a position inside that document. It is up to the client to decide how a selection is converted into a position when issuing a request for a text document. The client can, for example, honor or ignore the selection direction to make LSP request consistent with features implemented internally.
+A parameter literal used in requests to pass a text document and a
+position or optional range within that document.
+
+To be consistent with features implemented internally, the client is
+free to choose for each LSP request how a cursor position or text
+selection is converted into a position or range.
+For example, some editors display the cursor as a caret at the current
+insertion point between characters (`A|BC`), whereas others display it
+as a box around the character after the insertion point (`A🄱C`).
+Depending on the operation, in the absence of a selection range,
+editors of the second kind may choose to interpret the cursor position
+as an implicit selection of the character _after_ the position.
 
 ```typescript
 interface TextDocumentPositionParams {
@@ -12,8 +23,14 @@ interface TextDocumentPositionParams {
 	textDocument: TextDocumentIdentifier;
 
 	/**
-	 * The position inside the text document.
+	 * The cursor position within the text document.
 	 */
 	position: Position;
+
+	/**
+	 * The selected range within the text document, if any.
+	 * The position is typically one of the endpoints of the range.
+	 */
+	range?: Range;
 }
 ```