Remember file mask between search panels

[lewbei]

Fixes #37.

This persists the File mask field between Storm Search panels.

Changes:

• stores file mask updates in VS Code workspaceState
• injects the saved file mask into newly created webview panels
• initializes the webview search filter state from the restored input value
• automatically opens the filter row when a saved file mask exists
• escapes the saved value before putting it into the input attribute

Verification:

npm ci
npm run compile

npm run compile passes.

I also ran npm run lint, but the repository currently has no ESLint config file, so ESLint exits before linting source files:

ESLint couldn't find a configuration file.