Fix incorrect 'this' substitution in decorated static 'accessor' fields

[rbuckton]

The issue occurs because substitution assumes a this in a static PropertyDeclaration can only occur in an initializer, and part of performing this check includes looking at the original node for the declaration containing the this. However, an accessor is also transformed into a get/set pair that contains this, and those generated get and set methods will also have their original node set to the same PropertyDeclaration as the initializer, which results in overzealous substitution.

This PR changes the behavior to prevent substitution of the this keyword in the generated calls to __classPrivateFieldGet and __classPrivateFieldSet in the generated get and set methods.

Fixes #54473