feat(toml): highlight table headers

[branchvincent]

This highlights toml [table] headers differently, which matches GitHub (via linguist) and nvim-treesitter (although this implementation overlooks some combinations of quoted and dotted keys).

Before	After

And for reference:

GitHub markdown

title = "TOML Example"

[table]
name = "Tom Preston-Werner"

["table"."dotted"]
enabled = true
ports = [8000, 8001, 8002]
dotted.key = { cpu = 79.5, case = 72.0 }
"quoted.key" = [["delta", "phi"], [3.14]]

[[table.dotted.array]]
ip = "10.0.0.1"
role = "frontend"

[bare]
[dot.ted]
["quoted"]

[[bare]]
[[dot.ted]]
[["quoted"]]

[dead10ck]

This seems fine to me, thank you!