Make linkeable lines a link to themselves

Currently its already possible to make line lumbers linkeable. Getting
such a link however requires the end user to look at the pages source
and then to manually edit the URL, which is not a great UX.

This PR changes that to make the line numbers a link to themselves, so
clicking on them gives a link to that line that can then be passed
around, similiar to e.G. GitHub.

formatters/html/html_test.go

@@ -108,6 +108,32 @@ func TestTableLineNumberNewlines(t *testing.T) {
 </span>`)
 }
 
+func TestLinkeableLineNumbers(t *testing.T) {
+	f := New(WithClasses(true), WithLineNumbers(true), LinkableLineNumbers(true, "line"))
+	it, err := lexers.Get("go").Tokenise(nil, "package main\nfunc main()\n{\nprintln(\"hello world\")\n}\n")
+	assert.NoError(t, err)
+
+	var buf bytes.Buffer
+	err = f.Format(&buf, styles.Fallback, it)
+	assert.NoError(t, err)
+
+	assert.Contains(t, buf.String(), `id="line1"><a style="outline: none; text-decoration:none; color:inherit" href="#line1">1</a>`)
+	assert.Contains(t, buf.String(), `id="line5"><a style="outline: none; text-decoration:none; color:inherit" href="#line5">5</a>`)
+}
+
+func TestTableLinkeableLineNumbers(t *testing.T) {
+	f := New(WithClasses(true), WithLineNumbers(true), LineNumbersInTable(true), LinkableLineNumbers(true, "line"))
+	it, err := lexers.Get("go").Tokenise(nil, "package main\nfunc main()\n{\nprintln(`hello world`)\n}\n")
+	assert.NoError(t, err)
+
+	var buf bytes.Buffer
+	err = f.Format(&buf, styles.Fallback, it)
+	assert.NoError(t, err)
+
+	assert.Contains(t, buf.String(), `id="line1"><a style="outline: none; text-decoration:none; color:inherit" href="#line1">1</a>`)
+	assert.Contains(t, buf.String(), `id="line5"><a style="outline: none; text-decoration:none; color:inherit" href="#line5">5</a>`)
+}
+
 func TestTableLineNumberSpacing(t *testing.T) {
 	testCases := []struct {
 		baseLineNumber int