build: move all generator scripts to src/gen/

- Move all generator Lua scripts to the `src/gen/`
- Add a `.luarc.json` to `src/gen/`
- Add a `preload.lua` to `src/gen/`
  - Add `src` to `package.path` so it aligns with `.luarc.json'
- Fix all `require` statements in `src/gen/` so they are consistent:
    - `require('scripts.foo')` -> `require('gen.foo')`
    - `require('src.nvim.options')` -> `require('nvim.options')`
    - `require('api.dispatch_deprecated')` -> `require('nvim.api.dispatch_deprecated')`

src/gen/gen_char_blob.lua

@@ -0,0 +1,96 @@
+if arg[1] == '--help' then
+  print('Usage:')
+  print('  ' .. arg[0] .. ' [-c] target source varname [source varname]...')
+  print('')
+  print('Generates C file with big uint8_t blob.')
+  print('Blob will be stored in a static const array named varname.')
+  os.exit()
+end
+
+-- Recognized options:
+--   -c   compile Lua bytecode
+local options = {}
+
+while true do
+  local opt = string.match(arg[1], '^-(%w)')
+  if not opt then
+    break
+  end
+
+  options[opt] = true
+  table.remove(arg, 1)
+end
+
+assert(#arg >= 3 and (#arg - 1) % 2 == 0)
+
+local target_file = arg[1] or error('Need a target file')
+local target = io.open(target_file, 'w')
+
+target:write('#include <stdint.h>\n\n')
+
+local index_items = {}
+
+local warn_on_missing_compiler = true
+local modnames = {}
+for argi = 2, #arg, 2 do
+  local source_file = arg[argi]
+  local modname = arg[argi + 1]
+  if modnames[modname] then
+    error(string.format('modname %q is already specified for file %q', modname, modnames[modname]))
+  end
+  modnames[modname] = source_file
+
+  local varname = string.gsub(modname, '%.', '_dot_') .. '_module'
+  target:write(('static const uint8_t %s[] = {\n'):format(varname))
+
+  local output
+  if options.c then
+    local luac = os.getenv('LUAC_PRG')
+    if luac and luac ~= '' then
+      output = io.popen(luac:format(source_file), 'r'):read('*a')
+    elseif warn_on_missing_compiler then
+      print('LUAC_PRG is missing, embedding raw source')
+      warn_on_missing_compiler = false
+    end
+  end
+
+  if not output then
+    local source = io.open(source_file, 'r')
+      or error(string.format("source_file %q doesn't exist", source_file))
+    output = source:read('*a')
+    source:close()
+  end
+
+  local num_bytes = 0
+  local MAX_NUM_BYTES = 15 -- 78 / 5: maximum number of bytes on one line
+  target:write(' ')
+
+  local increase_num_bytes
+  increase_num_bytes = function()
+    num_bytes = num_bytes + 1
+    if num_bytes == MAX_NUM_BYTES then
+      num_bytes = 0
+      target:write('\n ')
+    end
+  end
+
+  for i = 1, string.len(output) do
+    local byte = output:byte(i)
+    target:write(string.format(' %3u,', byte))
+    increase_num_bytes()
+  end
+
+  target:write('  0};\n')
+  if modname ~= '_' then
+    table.insert(
+      index_items,
+      '  { "' .. modname .. '", ' .. varname .. ', sizeof ' .. varname .. ' },\n\n'
+    )
+  end
+end
+
+target:write('static ModuleDef builtin_modules[] = {\n')
+target:write(table.concat(index_items))
+target:write('};\n')
+
+target:close()