Add missing .js extensions to remaining imports

[mxxk]

Also, add ESLint rules to flag these automatically moving forward.

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Since this package is declared as an ES module,

structured-headers/package.json

Line 5 in 375fff6

"type": "module",

when structured-headers is consumed as a dependency, TypeScript expects imports (including those in .d.ts files) to have the .js extension:

node_modules/structured-headers/dist/types.d.ts:1:23 - error TS2835: Relative import paths need explicit file extensions in ECMAScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Did you mean './token.js'?

1 import { Token } from './token';
                        ~~~~~~~~~

node_modules/structured-headers/dist/types.d.ts:2:31 - error TS2835: Relative import paths need explicit file extensions in ECMAScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Did you mean './displaystring.js'?

2 import { DisplayString } from './displaystring';
                                ~~~~~~~~~~~~~~~~~

node_modules/structured-headers/dist/util.d.ts:1:33 - error TS2835: Relative import paths need explicit file extensions in ECMAScript imports when '--moduleResolution' is 'node16' or 'nodenext'. Did you mean './types.js'?

1 import { Item, InnerList } from './types';
                                  ~~~~~~~~~

The .js extension is inserted automatically in emitted .js files, but not in emitted .d.ts files.

Most imports already have a .js extension, for example,

• structured-headers/src/index.ts

  Line 1 in 375fff6

  export * from './serializer.js';

• structured-headers/src/parser.ts

  Line 10 in 375fff6

  import { Token } from './token.js';

so this PR is making the remaining imports consistent.

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See: microsoft/TypeScript#50482.

[evert]

Thank you! Great PR + appreciate the linting changes