chore: add more concrete types to `loot-core` (#1186)

...s/desktop-client/src/components/settings/Experimental.tsx

packages/desktop-client/src/hooks/useFeatureFlag.ts

packages/desktop-client/src/style/theme.tsx

packages/loot-core/src/client/state-types/prefs.d.ts

packages/loot-core/src/mocks/budget.ts

packages/loot-core/src/server/accounts/parse-file.ts

packages/loot-core/src/server/api.ts

packages/loot-core/src/server/budget/actions.ts

packages/loot-core/src/server/main.test.ts

packages/loot-core/src/server/main.ts

packages/loot-core/src/server/prefs.ts

packages/loot-core/src/server/server-config.ts

packages/loot-core/src/server/sheet.ts

packages/loot-core/src/server/spreadsheet/globals.ts

packages/loot-core/src/server/spreadsheet/spreadsheet.ts

packages/loot-core/src/server/spreadsheet/util.ts

packages/loot-core/src/server/sync/encoder.ts

packages/loot-core/src/server/sync/index.ts

packages/loot-core/src/shared/months.ts

packages/loot-core/src/shared/util.ts