save auto-escaping of generated URLs when possible for performance reasons
[TwigBridge] added some unit test for the previous commit
merged branch fabpot/twig-url-escaping (PR #7850)

This PR was merged into the master branch.

Discussion
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[2.3] [TwigBridge] save auto-escaping of generated URLs when possible

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes: optimization]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #7088
| License       | MIT
| Doc PR        | [-]

Determines at compile time whether the generated URL will be safe and thus
saving the unneeded automatic escaping for performance reasons.

The URL generation process percent encodes non-alphanumeric characters. So there is no risk
that malicious/invalid characters are part of the URL. The only character within an URL that
must be escaped in html is the ampersand ("&") which separates query params. So we cannot mark
the URL generation as always safe, but only when we are sure there won't be multiple query
params. This is the case when there are none or only one constant parameter given.
E.g. we know beforehand this will be safe:
- path('route')
- path('route', {'param': 'value'})

But the following may not:
- path('route', var)
- path('route', {'param': ['val1', 'val2'] }) // a sub-array
- path('route', {'param1': 'value1', 'param2': 'value2'})

If param1 and param2 reference placeholder in the route, it would still be safe. But we don't know.

Commits
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725568b [TwigBridge] added some unit test for the previous commit
0721ff8 save auto-escaping of generated URLs when possible for performance reasons