Add an opt-in option to lowercase filter values

Filter values that only differ in case, like `Mint` and `mint`, came
through as separate values. The new `--lowercase-filters` option
(`lowercase_filters`, `PAGEFIND_LOWERCASE_FILTERS`, and
`lowercaseFilters` / `lowercase_filters` in the Node and Python
wrappers) lowercases filter values when indexing and merges values on
a page that become equal. Filter names are left as they are. Off by
default.

Fixes #737

wrappers/node/lib/index.js

@@ -59,6 +59,7 @@ export const createIndex = (config) => new Promise((resolve, reject) => {
                 keep_index_url: config?.keepIndexUrl,
                 write_playground: config?.writePlayground,
                 include_characters: config?.includeCharacters,
+                lowercase_filters: config?.lowercaseFilters,
             }
         }, (response) => {
             /** @type {function(InternalResponsePayload): Omit<NewIndexResponse, 'errors'>?} */