Is there an enum equivalent to tonumber() ?

This is a hacky way and I would not recommend this. If you want to though then here's a solution.

function toEnum(s)
   local t = {}
   if not s:match("^Enum") then -- if not an enum then stop the function 
      return
   end
   for match in s:gmatch("[^%.]+") do
      table.insert(t, match) --[[
          split the string into different parts e.g 
          {"Enum", "KeyCode", "LeftControl"}
          ]]
   end
   local function recurse(p, ...) --loop through table elements and index each recursively 
      if getfenv()[p] then --grab the Enum global
         p = getfenv()[p]
      end
      if select('#', ...) > 1 then --if there is still something to be indexed
         return recurse(p[...], select('2', ...))
      else
         return p[...] -- when it is done looping, return the final value
      end
   end
   local ret = recurse(unpack(t))
   return ret 
end

Here's my solution, since the others' are hacky or otherwise unnecessarily complicated.

local function toEnum(str)
    local enum = Enum

    str = str:gsub("^Enum.", "")

    for part in str:gmatch("[^.]+") do
        enum = enum[part]
    end

    return enum
end

Note that the "Enum." prefix will be optional.

In its raw form an Enum is just a value but this will get very confussing for programmers if we do not assign meaning to what the value is.

An Enum is an object containing three pieces of information Name, Value and EnumType. More imformation can be found here.

In your case we can use the name if you build a key value pair with the Enum itself being the value.

Example:-

local keyNames = {} -- store the keyName = Enum
for _, key in pairs(Enum.KeyCode:GetEnumItems()) do -- gets the list of enums for keys
    keyNames[key.Name] = key  -- map key name to the enum
end

print(keyNames['LeftControl'])

Hope this helps.