perf: reduce extra allocation at WebViewBuilderExtWindows::with_additional_browser_args argument

[rhysd]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue perf: reduce extra allocation at WebViewBuilderExtWindows::with_additional_browser_args argument #783
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

Currently WebViewBuilderExtWindows::with_additional_browser_args argument is typed with AsRef<str>. However the method converts the string slice into string buffer with heap allocation. This causes redundant allocation when String value is passed.

let args: String = ...; // We already have heap-allocated string buffer
builder = builder.with_additional_browser_args(args); // New buffer is allocated

This PR fixes the point.

Strictly speaking, changing from AsRef<str> to Into<String> is a breaking change, but it is very small because major string types like &str, String, and Cow<'a, str> can still be passed to the argument.

[amrbashir]

Very cool, thank you!