This is mostly plumbing. More relevant changes for this are in a PR for the native component.

• Indicates in a configuration if values were explicitly provided by the user. (Otherwise, they are derived from defaults or other implicit behavior, which would not warrant disturbing the user with a warning).
• Adds some strings for warning/logging (open to input on these).
• Changed c18/gnu18 to c17/gnu17.
• Removed prior fix for detecting compiler filename in environment path. This is now handled on the native side for all config types.

When combined with the corresponding native PR, addresses:

• Search for compile_commands.json compiler path in the PATH environment variable #6179
• Change cStandard enum values c18 and gnu18 to c17 and gnu17 #6105
• Mixed up default intellisense options #6075
  (others)

I will also have a PR for cpptools-api to make IntelliSenseMode optional from custom config providers, as we can now fully auto-detect it (and don't need to require all custom config providers to replicate that behavior).