add TrimPrefix method to go along with TrimSuffix

Updates #10

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

golink.go

@@ -651,6 +651,7 @@ func (e expandEnv) User() (string, error) {
 var expandFuncMap = texttemplate.FuncMap{
 	"PathEscape":  url.PathEscape,
 	"QueryEscape": url.QueryEscape,
+	"TrimPrefix":  strings.TrimPrefix,
 	"TrimSuffix":  strings.TrimSuffix,
 	"Match":       regexMatch,
 }

golink_test.go

@@ -411,6 +411,12 @@ func TestExpandLink(t *testing.T) {
 			remainder: "a/b+c",
 			want:      "http://host.com/a%2Fb%2Bc",
 		},
+		{
+			name:      "template-with-trimprefix-func",
+			long:      `http://host.com/{{TrimPrefix .Path "BUG-"}}`,
+			remainder: "BUG-123",
+			want:      "http://host.com/123",
+		},
 		{
 			name:      "template-with-trimsuffix-func",
 			long:      `http://host.com/{{TrimSuffix .Path "/"}}`,

tmpl/help.html

@@ -63,6 +63,7 @@ <h2 id="advanced">Advanced destination links</h2>
 <ul>
   <li><code>PathEscape</code> is the <a href="https://pkg.go.dev/net/url#PathEscape">url.PathEscape</a> function for escaping values inside a URL path.
   <li><code>QueryEscape</code> is the <a href="https://pkg.go.dev/net/url#QueryEscape">url.QueryEscape</a> function for escaping values inside a URL query.
+  <li><code>TrimPrefix</code> is the <a href="https://pkg.go.dev/strings#TrimPrefix">strings.TrimPrefix</a> function for removing a leading prefix.
   <li><code>TrimSuffix</code> is the <a href="https://pkg.go.dev/strings#TrimSuffix">strings.TrimSuffix</a> function for removing a trailing suffix.
   <li><code>Match</code> is the <a href="https://pkg.go.dev/regexp#MatchString">regexp.MatchString</a> function for matching a regular expression pattern.
 </ul>