Removes arguments with leading _ from unused variable lint check.

This allows `_` to be used as a prefix for unused variable names, which is a common annotation.

ESLint supports different options for variables, arguments, catch clauses, and destructured array. I opted to only allow this annotation for arguments and destructures because IMHO:
* Unused variables are extremely rare, only ocurring in `@rules_prerender` in type-only tests. Those variables are usually load bearing in a unique way and deserve an actual ignore comment, otherwise the meaning of the leading `_` is unclear.
* Catch clauses already support optional names, you can do `try {} catch {}`, so if the error is unused, the name should be omitted entirely.

Author: dgp1130

.eslintrc.yaml

@@ -40,6 +40,12 @@ rules:
   no-constant-condition:
   - error
   - checkLoops: false # `while (true) { /* ... */ }` is ok.
+  no-unused-vars:
+  - error
+  - argsIgnorePattern: "^_"
+    destructuredArrayIgnorePattern: "^_"
+    # Deliberately _not_ setting `varsIgnorePattern`, since that benefits from
+    # an explicit ignore comment instead of a load-bearing `_`.
 
 # Enable TypeScript rules only for `*.[cm]?ts` files.
 overrides:
@@ -62,6 +68,13 @@ overrides:
     no-undef:
       - off
 
+    '@typescript-eslint/no-unused-vars':
+    - error
+    - argsIgnorePattern: "^_"
+      destructuredArrayIgnorePattern: "^_"
+      # Deliberately _not_ setting `varsIgnorePattern`, since that benefits from
+      # an explicit ignore comment instead of a load-bearing `_`.
+
 - files:
   - "**/*_test.ts"
   - "**/test.ts"

common/file_system_fake.mts

@@ -27,7 +27,6 @@ export class FileSystemFake implements FileSystem {
         this.fs.set(dest, contents);
     }
 
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     public async mkdir(_dir: string, _options?: { recursive?: boolean }):
             Promise<void> {
         // Nothing to do.
@@ -37,7 +36,6 @@ export class FileSystemFake implements FileSystem {
     readFile(path: string, options: { encoding: 'utf8' }): Promise<string>;
     readFile(path: string, options?: string | { encoding?: string }):
         Promise<string | Buffer>;
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     public async readFile(inputPath: string, _options?: string | { encoding?: string }): Promise<string | Buffer> {
         const filePath = path.normalize(inputPath);
         const content = this.fs.get(filePath);