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/api/keysets_defs.h

@@ -179,6 +179,10 @@ typedef struct {
   Boolean italic;
   Boolean reverse;
   Boolean altfont;
+  Boolean dim;
+  Boolean blink;
+  Boolean conceal;
+  Boolean overline;
   Boolean nocombine;
   Boolean default_ DictKey(default);
   DictAs(highlight_cterm) cterm;
@@ -212,6 +216,10 @@ typedef struct {
   Boolean italic;
   Boolean reverse;
   Boolean altfont;
+  Boolean dim;
+  Boolean blink;
+  Boolean conceal;
+  Boolean overline;
   Boolean nocombine;
 } Dict(highlight_cterm);
 

src/nvim/api/vim.c

@@ -152,6 +152,11 @@ DictAs(get_hl_info) nvim_get_hl(Integer ns_id, Dict(get_highlight) *opts, Arena
 ///                - sp: color name or "#RRGGBB"
 ///                - blend: integer between 0 and 100
 ///                - bold: boolean
+///                - altfont: boolean
+///                - blink: boolean
+///                - dim: boolean
+///                - conceal: boolean
+///                - overline: boolean
 ///                - standout: boolean
 ///                - underline: boolean
 ///                - undercurl: boolean