feat(highlight): support more SGR attributes #37901

Problem:
TUI does not support several standard SGR text attributes:
- dim/faint (SGR 2)
- blink (SGR 5)
- conceal (SGR 8)
- overline (SGR 53)
This means that when a program running in the embedded terminal emits
one of these escape codes, we drop it and don't surface it to the
outer terminal.

Solution:
- Add support for those attributes.
- Also add corresponding flags to `nvim_set_hl` opts, so users can set
  these attributes in highlight groups.
  - refactor(highlight): widen `HlAttrFlags` from `int16_t` to `int32_t`
    Widen the `rgb_ae_attr` and `cterm_ae_attr` fields in HlAttrs from
    int16_t to int32_t to make room for new highlight attribute flags,
    since there was only one spare bit left.
  - The C flag is named HL_CONCEALED to avoid colliding with the
    existing HL_CONCEAL in syntax.h (which is a syntax group flag, not
    an SGR attribute).
- Also note that libvterm doesn't currently support the dim and overline
  attributes, so e.g. `printf '\e[2mThis should be dim\n'` and `printf
  '\e[53mThis should have an overline\n'` are still not rendered
  correctly when run from the embedded terminal.

runtime/lua/vim/_meta/vimfn.lua

@@ -10078,6 +10078,10 @@ function vim.fn.synID(lnum, col, trans) end
 --- "strikethrough"  "1" if struckthrough
 --- "altfont"  "1" if alternative font
 --- "nocombine"  "1" if nocombine
+--- "dim"  "1" if half-bright/dimmed
+--- "blink"  "1" if blinking
+--- "conceal"  "1" if concealed
+--- "overline"  "1" if overlined
 ---
 --- Returns an empty string on error.
 ---