refactor(options): replace p_force_(on|off) with immutable (#26209)

Problem: We use the `p_force_on` and `p_force_off` variables to check if a variable is immutable and what its default value is. This is not only hacky and unintuitive, but also is limited to only boolean options.

Solution: Replace `p_force_on` and `p_force_off` with an `immutable` property for options, which indicates if an option is immutable. Immutable options cannot be changed from their default value.

Ref: #25672.

src/nvim/generators/gen_options.lua

@@ -143,9 +143,13 @@ local function dump_option(i, o)
   end
   if o.varname then
     w('    .var=&' .. o.varname)
+  -- Immutable options should directly point to the default value
+  elseif o.immutable then
+    w(('    .var=&options[%u].def_val'):format(i - 1))
   elseif #o.scope == 1 and o.scope[1] == 'window' then
     w('    .var=VAR_WIN')
   end
+  w('    .immutable=' .. (o.immutable and 'true' or 'false'))
   if #o.scope == 1 and o.scope[1] == 'global' then
     w('    .indir=PV_NONE')
   else