cmd/tsconnect: prefetch main.wasm when serving

Avoids waterfalling of requests from the file (its load is triggered
from JavaScript).

Also has other cleanups to index.html, adding a <title> and moving the
<script> to being loaded sooner (but still not delaying page rendering
by using the defer attribute).

Fixes #5141
Fixes #5135

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/common.go

@@ -119,6 +119,9 @@ func runYarn(args ...string) error {
 // from entry points to hashed file names.
 type EsbuildMetadata struct {
 	Outputs map[string]struct {
+		Inputs map[string]struct {
+			BytesInOutput int64 `json:"bytesInOutput"`
+		} `json:"inputs,omitempty"`
 		EntryPoint string `json:"entryPoint,omitempty"`
 	} `json:"outputs,omitempty"`
 }