feat(font): add iosevka (@leana8959) (#6674)

### Description

<!-- Please describe the change(s) made in your PR -->
- added woff2 font file
- updated `frontend/static/{fonts/_list.json,styles/fonts.scss}` files

- tested locally, the font looks as such:

![image](https://github.com/user-attachments/assets/5a853477-335f-4c9e-b845-589f300243d4)


### Checks

- [ ] Adding quotes?
- [ ] Make sure to include translations for the quotes in the
description (or another comment) so we can verify their content.
- [ ] Adding a language?
- Make sure to follow the [languages
documentation](https://github.com/monkeytypegame/monkeytype/blob/master/docs/LANGUAGES.md)
  - [ ] Add language to `packages/contracts/src/schemas/languages.ts`
- [ ] Add language to exactly one group in
`frontend/src/ts/constants/languages.ts`
  - [ ] Add language json file to `frontend/static/languages` 
- [ ] Adding a theme?
- Make sure to follow the [themes
documentation](https://github.com/monkeytypegame/monkeytype/blob/master/docs/THEMES.md)
  - [ ] Add theme to `packages/contracts/src/schemas/themes.ts`
  - [ ] Add theme to `frontend/src/ts/constants/themes.ts`
  - [ ] Add theme css file to `frontend/static/themes`
- Also please add a screenshot of the theme, it would be extra awesome
if you do so!
- [ ] Adding a layout?
- [ ] Make sure to follow the [layouts
documentation](https://github.com/monkeytypegame/monkeytype/blob/master/docs/LAYOUTS.md)
  - [ ] Add layout to `packages/contracts/src/schemas/layouts.ts`
  - [ ] Add layout json file to `frontend/static/layouts` 
- [ ] Check if any open issues are related to this PR; if so, be sure to
tag them below.
- [x] Make sure the PR title follows the Conventional Commits standard.
(https://www.conventionalcommits.org for more info)
- [x] Make sure to include your GitHub username prefixed with @ inside
parentheses at the end of the PR title.

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frontend/src/styles/fonts.scss

@@ -107,6 +107,9 @@ $fonts: (
   "Geist Mono": (
     "src": "GeistMono-Medium",
   ),
+  "Iosevka": (
+    "src": "Iosevka-Regular",
+  ),
 );
 
 $font-defauls: (

frontend/static/fonts/_list.json

@@ -114,5 +114,8 @@
   },
   {
     "name": "Geist Mono"
+  },
+  {
+    "name": "Iosevka"
   }
-]
+]