Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

- call-non-callable
- missing-fields
- redefined-label
- redefined-local
- undefined-field
- unreachable-code

Promote redefined-local and unreachable-code to warnings.

Fix DOS line-ending handling for multiline semantic tokens, and support
intersection types in the help parser.

AI-assisted

benchmark: https://gist.github.com/ofseed/6224529d77c016c36f7ab2f977059848

    local rounds = tonumber(arg[1]) or 1000
    local count = tonumber(arg[2]) or 1000

    -- Load the table.clear function.
    local clear = require("table.clear")

    local function fill(t, n)
      for i = 1, n do
        t[i] = i
      end
    end

    local function bench_reassign(n_rounds, n_items)
      local t = {}
      local start = os.clock()

      for _ = 1, n_rounds do
        t = {}
        collectgarbage("collect")
        fill(t, n_items)
      end

      return os.clock() - start
    end

    local function bench_reassign_no_gc(n_rounds, n_items)
      local t = {}
      local start = os.clock()

      for _ = 1, n_rounds do
        t = {}
        fill(t, n_items)
      end

      return os.clock() - start
    end

    local function bench_clear(n_rounds, n_items)
      local t = {}
      local start = os.clock()

      for _ = 1, n_rounds do
        clear(t)
        fill(t, n_items)
      end

      return os.clock() - start
    end

    -- Warm up LuaJIT before the real benchmark.
    do
      local t = {}
      for _ = 1, 2000 do
        clear(t)
        fill(t, count)
      end
    end

    collectgarbage("collect")

    local reassign_time = bench_reassign(rounds, count)
    collectgarbage("collect")

    local reassign_no_gc_time = bench_reassign_no_gc(rounds, count)
    collectgarbage("collect")

    local clear_time = bench_clear(rounds, count)

    print(string.format("rounds=%d count=%d", rounds, count))
    print(string.format("t = {} + GC   : %.6f s", reassign_time))
    print(string.format("t = {}        : %.6f s", reassign_no_gc_time))
    print(string.format("table.clear   : %.6f s", clear_time))
    print(string.format("vs + GC       : %.2fx", reassign_time / clear_time))
    print(string.format("vs no GC      : %.2fx", reassign_no_gc_time / clear_time))

benchmark result:

    rounds=1000 count=1000
    t = {} + GC   : 0.022469 s
    t = {}        : 0.002570 s
    table.clear   : 0.000387 s
    vs + GC       : 58.06x
    vs no GC      : 6.64x

`count` is how many items the table has, and `round` is how many rounds we fill
the table, clear, and then refill it. `table = {}` is clear the table by
resigning a new empty one, because this script does not run persistently like
nvim so GC is not triggered, so I added another extreme control group that
manually triggers GC.