add TrimSuffix to the function map

[vielmetti]

Adds strings.TrimSuffix to the function map, enabling it to be used in URL definitions.

This accompanies the discussion in #9 and is motivated by the browser misfeature of my Safari instance, which fails to recognize a local domain URL as a URL unless it has a trailing slash. The intended use of this function is to trim that trailing slash.

(There is most certainly some more elegant way to do this, but as a start this is the one-line fix suggested by @willnorris that lets me make forward progress.)

[willnorris]

Would you mind also adding a test case in https://github.com/tailscale/golink/blob/main/golink_test.go ? I'm also happy to add one myself.

[willnorris]

(in those tests, the leftover path is called "remainder". I just never renamed that apparently)

[willnorris]

hmm... I dont' know if I can suggest changes on a file that you didn't already touch, so the additional test case would just be:

		{
			name:      "template-with-trimsuffix-func",
			long:      `http://host.com/{{TrimSuffix .Path "/"}}`,
			remainder: "a/",
			want:      "http://host.com/a",
		},

[vielmetti]

Test added as requested.

[willnorris]

Awesome, thanks! Looks great.