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

pagefind/integration_tests/filtering/filter-values-can-be-lowercased.toolproof.yml

@@ -0,0 +1,51 @@
+name: Filtering > Filter values can be lowercased
+steps:
+  - step: I have the environment variable "PAGEFIND_SITE" set to "public"
+  - step: I have a "public/index.html" file with the content {html}
+    html: >-
+      <!DOCTYPE html><html lang="en"><head></head><body><p
+      data-results>Nothing</p></body></html>
+  - step: I have a "public/cheeka/index.html" file with the content {html}
+    html: >-
+      <!DOCTYPE html><html lang="en"><head></head><body><span
+      data-pagefind-filter="color">Black</span>
+
+      <span data-pagefind-filter="color">White</span>
+
+      <span data-pagefind-filter="color">white</span>
+
+      <h1>Cat</h1></body></html>
+  - step: I have a "public/theodore/index.html" file with the content {html}
+    html: >-
+      <!DOCTYPE html><html lang="en"><head></head><body><span
+      data-pagefind-filter="color">WHITE</span>
+
+      <h1>Cat</h1></body></html>
+  - macro: I run Pagefind with "--lowercase-filters"
+  - step: stdout should contain "Running Pagefind"
+  - step: I serve the directory "public"
+  - step: In my browser, I load "/"
+  - step: In my browser, I evaluate {js}
+    js: |-
+      let pagefind = await import("/pagefind/pagefind.js");
+
+      let filters = await pagefind.filters();
+      let strings = Object.entries(filters).map(([filter, values]) => {
+          values = Object.entries(values).map(([value, count]) => {
+              return `${value}(${count})`;
+          })
+          return `${filter}:[${values.join(", ")}]`;
+      });
+
+      let search = await pagefind.search("Cat", { filters: { color: "white" } });
+      let data = await Promise.all(search.results.map(result => result.data()));
+
+      document.querySelector('[data-results]').innerText = strings.join(' ')
+          + ' ' + data.map(d => d.url).sort().join(', ');
+  - step: In my browser, the console should be empty
+  - step: In my browser, I evaluate {js}
+    js: >-
+      let val = await toolproof.querySelector("[data-results]");
+
+      toolproof.assert_eq(val.innerHTML, `color:[black(1), white(2)]
+      /cheeka/, /theodore/`);

pagefind/integration_tests/node_api/node_base/pagefind-lowercase-filters-config.toolproof.yml

@@ -0,0 +1,39 @@
+name: Node API Base Tests > Pagefind lowercaseFilters config
+platforms:
+  - linux
+  - mac
+
+steps:
+  - ref: ./background.toolproof.yml
+  - step: I have a "public/index.js" file with the content {js}
+    js: |2-
+       import * as pagefind from "pagefind";
+
+       const run = async () => {
+           const { index } = await pagefind.createIndex({
+               lowercaseFilters: true,
+           });
+           await index.addHTMLFile({sourcePath: "page_a/index.html", content: "<html lang='en'><body><h1 data-pagefind-filter='color:White'>Cat</h1></body></html>"});
+           await index.addCustomRecord({
+               url: "/page_b/",
+               content: "Cat",
+               language: "en",
+               filters: { color: ["WHITE", "Black"] },
+           });
+           await index.writeFiles();
+           console.log(`Successfully wrote files`);
+       }
+
+       run();
+  - macro: I run Pagefind Node as "index.js" in "public"
+  - step: stdout should contain "Successfully wrote files"
+  - step: The file "public/pagefind/pagefind.js" should not be empty
+  - step: I serve the directory "public"
+  - step: In my browser, I load "/"
+  - step: In my browser, I evaluate {js}
+    js: |-
+      let pagefind = await import("/pagefind/pagefind.js");
+      let filters = await pagefind.filters();
+
+      toolproof.assert_eq(JSON.stringify(filters), `{"color":{"black":1,"white":2}}`);
+  - step: In my browser, the console should be empty