This updates the extmark namespace to search for when filtering out
semantic tokens to match the new namespace style recently introduced.

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.

This mostly means replacing `nvim_buf_call` and `nvim_win_call` with
`vim._with`.

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.