Dynamic registration of file watches

[toinehartman]

Is your feature request related to a problem? Please describe.

Currently, the parametric language server registers watches for files with any extension since, at initialization time, the extensions for registered languages are not yet known. When a notification for a file event then comes in, it is first checked to have an extension that is registered, before delegating to the relevant language implementation if applicable.

Ergo: a lot of redundant file events.

Describe the solution you'd like

Since the addition of dynamic capability registration (in #706), it should be possible to register file watches dynamically, updating the registration whenever the set of extensions of registered languages changes.

Describe alternatives you've considered

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Additional context

Related to #955.