fix(FileSettingsCache): Always watch repo-local settings

[mstv]

Fixes #11720

Proposed changes

Cleanup FileSettingsCache:

• Always watch the settings file also if the checkout does not contain that file on startup (repo open)
• Hence remove _canEnableFileWatcher
• React on deletion, too
• There is no need to distinguish the kind of file change because we just need to know whether to refresh the settings or not
• Use FileChanged instead of assigning _lastFileModificationDate

Screenshots

N/A

Test methodology

• manual
• Remove obsolete testcase ctor_CanEnableFileWatcher_should_be_false_if_invalid_dir which is redundant with the existing
  ctor_FileWatcher_EnableRaisingEvents_should_be_false_if_invalid_dir

Merge strategy

I agree that the maintainer squash merge this PR (if the commit message is clear).

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✒️ I contribute this code under The Developer Certificate of Origin.

[mstv]

I have added the handler to the Deleted event amongst others, @astos-marcb.

[astos-marcb]

@mstv preliminary tests with b89f89d CI artifact suggest both issues are fixed now.
Thanks for the quick fix, I'll try to use less strange repository setups in the future. 😄

[gerhardol]

Have not run