Fix Lua keywords

[E-Fir]

Problem: Lua keywords don't have recognized properly. They are treated as identifiers.
Example:
Source to parse:

if true and not false or nil then
  repeat
    break
  until true
elseif 1 then
  local a = 1
else
  for k, v in pairs(_G) do
    local f = function()  end
  end
end

while true do end

return 1

Parsing:

for kind, text in lxsh.lexers.lua.gmatch(source) do
  if kind ~= 'whitespace' then
    print(kind, text)
  end
end

Result:

keyword if
constant true
operator and
operator not
constant false
operator or
constant nil
identifier then
identifier repeat
identifier break
identifier until
constant true
identifier elseif
number 1
identifier then
identifier local
identifier a
operator =
number 1
identifier else
identifier for
identifier k
operator ,
identifier v
identifier in
identifier pairs
operator (
identifier _G
operator )
identifier do
identifier local
identifier f
operator =
identifier function
operator (
operator )
identifier end
identifier end
identifier end
identifier while
constant true
identifier do
identifier end
identifier return
number 1

Only the first if is recognized properly.

Solution: fix the keyword pattern.