Reload the LS when relevant config changes

[jribbink]

Closes #494

Description

• Some refactoring to the Settings class (it was kind of bloated and all of the stuff for missing values, defaults, etc. is useless since it's handled by VSCode itself). The singleton was also kind of useless because we were injecting instances everywhere anyways and only complicates testing
• Adds a reload to LS when LS-related configuration changes (important for Cadence 1.0, since users will be changing cadence.flowCommand setting)
• Changes test concurrency setting to test.maxConcurrency instead. This was an oversight when I originally added the feature, it probably makes sense to start getting in the habit of namespacing properly.

Imo if specific settings require additional transformations, etc. we can move these to their own sub-providers but the top-level settings provider should be more generalized

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[nialexsan]

lgtm