Support `:let=<<` with leading spaces before EOF

[lehmacdj]

Expert from the start of :h :let=<<:

						*:let=<<* *:let-heredoc*
						*E990* *E991* *E172* *E221*
:let {var-name} =<< [trim] {endmarker}
text...
text...
{endmarker}

This feature has been supported by vim/neovim since around 2019, it would be good if it could be supported by this library as well.

Note: I came here because this isn't parsed by vint and I'm assuming that they're using a fairly up to date version of this library. I did check issues / pull requests here and didn't see anything about this though.

[lehmacdj]

vim/vim@f5842c5
neovim/neovim#11077

[lehmacdj]

Reduced scope of this, because I realized that let =<< is already implemented for the case where EOF is at the start of the line.