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.

src/nvim/eval.lua

@@ -9681,7 +9681,7 @@ M.funcs = {
 
       The optional argument {opts} is a Dict and supports the following items:
 
-        peer  : If |TRUE|, servers not started by |serverstart()| 
+        peer  : If |TRUE|, servers not started by |serverstart()|
                 will also be returned. (default: |FALSE|)
                 Not supported on Windows yet.
 
@@ -12158,6 +12158,10 @@ M.funcs = {
       "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.