Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.

Font-family names must be enclosed in quotation marks to ensure that
fonts are applied correctly when there are spaces in the name.

Fix an issue where multiple fonts specified in `vim.o.guifont` are
inserted as a single element, treating them as a single font.

Support for escaping commas with backslash and ignoring spaces
after a comma.

ref `:help 'guifont'`

Problem:
Things like underlines are always given a default foreground highlight
regardless of the value of `sp`.

Solution:
Check for `sp` first, and apply that color to the text decoration color if it
exists.

Limitations:
If there is no value of `sp`, vim applies a text decoration color that matches
the foreground of the text. This is still not implemented (and seems like a much
more complex problem): in TOhtml, the underline will still be given a default
foreground highlight.

Problem: :TOhtml doesn't properly handle virtual text when it has
multiple highlight groups. It also improperly calculates position offset
for multi-byte virt_text characters.

Solution: Apply the `vim.api.nvim_strwidth` broadly to properly
calculate character offset, and handle the cases where the `hl` argument
can be a table of multiple hl groups.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.