DEV: Preserve leading trivia when dts-generator replaces a node (#43282)

processTree() in dts-generator (used to bundle each vendored package's
.d.ts into @discourse/types) replaces certain nodes, like the `declare`
keyword, with an empty string. TypeScript scans a node's `pos` to
include its leading trivia (blank lines, comments), but skip() jumps
straight to `node.end` on replacement, so that trivia got silently
deleted along with the token being replaced.

This surfaced concretely in @floating-ui/dom's type declarations, where
`export { Axis }` was immediately followed by a blank line and `declare
type BivariantCallback = ...`. Removing `declare` also removed the blank
line, merging the two statements onto a single line and producing
invalid TypeScript in the generated
external-types/floating-ui__dom/index.d.ts (confirmed by PR #43280,
which bumped to a build containing this and failed `pnpm lint:types`
with `TS1005: ';' expected`).

The fix emits the node's leading trivia before applying the replacement,
so only the token itself is swallowed.

frontend/discourse-types/dts-generator.ts

@@ -104,6 +104,8 @@ function processTree(
     const replacement = replacer(node);
 
     if (replacement != null) {
+      const start = node.getStart(sourceFile);
+      code += sourceFile.text.slice(cursorPosition, start);
       code += replacement;
       skip(node);
     } else {