Add inline shortcode support

An inline shortcode's name must end with `.inline`, all lowercase.

E.g.:

```bash
{{< time.inline >}}{{ now }}{{< /time.inline >}}
```

The above will print the current date and time.

Note that an inline shortcode's inner content is parsed and executed as a Go text template with the same context as a regular shortcode template.

This means that the current page can be accessed via `.Page.Title` etc. This also means that there are no concept of "nested inline shortcodes".

The same inline shortcode can be reused later in the same content file, with different params if needed, using the self-closing syntax:

```
{{< time.inline />}}
```

Fixes #4011

common/herrors/file_error.go

@@ -92,7 +92,13 @@ func UnwrapFileError(err error) FileError {
 // with the given offset from the original.
 func ToFileErrorWithOffset(fe FileError, offset int) FileError {
 	pos := fe.Position()
-	pos.LineNumber = pos.LineNumber + offset
+	return ToFileErrorWithLineNumber(fe, pos.LineNumber+offset)
+}
+
+// ToFileErrorWithOffset will return a new FileError with the given line number.
+func ToFileErrorWithLineNumber(fe FileError, lineNumber int) FileError {
+	pos := fe.Position()
+	pos.LineNumber = lineNumber
 	return &fileError{cause: fe, fileType: fe.Type(), position: pos}
 }