feat($core): Use mkdtemp for default tempdir

[0az]

Summary

feat($core): use mkdtemp for default tempdir

This ensures that the defaults work under read-only node_modules. This
is the case in certain runtime environments (#723) as well as under Yarn
v2's Plug'n'Play (PNP).

This commit also implements best-effort cleanup of any such temporary
directories on process exit.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The exact location of the default temporary directory shouldn't break anything, as there exists wrappers in ctx (tempPath, etc).

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

• This was first proposed in part in Change temp directory #723, but was not implemented.

• This does not directly affect browser-visible output.

• Both EN/ZH config docs have been updated with the new default.

• I am not aware of any tests for this functionality. However, a manual run of the test suite shows that the temporary directories are cleaned up properly under normal operation.