feat(api): add filetype option nvim_get_option_value

- Also adjust the expr-mapping behaviour so normal commands and text
  changes are allowed in internal dummy buffers.

runtime/doc/api.txt

@@ -1969,6 +1969,10 @@ nvim_get_option_value({name}, {*opts})               *nvim_get_option_value()*
                 • win: |window-ID|. Used for getting window local options.
                 • buf: Buffer number. Used for getting buffer local options.
                   Implies {scope} is "local".
+                • filetype: |filetype|. Used to get the default option for a
+                  specific filetype. Cannot be used with any other option.
+                  Note: this is expensive, it is recommended to cache this
+                  value.
 
     Return: ~
         Option value

runtime/doc/news.txt

@@ -210,6 +210,9 @@ The following new APIs or features were added.
   https://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-injection .
   Support for the previous format will be removed in a future release.
 
+• |nvim_get_option_value()| now has a `filetype` option so it can return the
+  default option for a specific filetype.
+
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 CHANGED FEATURES                                                 *news-changes*