Centralize ThemeColor constants

ThemeColor string literals like `"problemsWarningIcon.foreground"` and `"notificationsErrorIcon.foreground"` are scattered across multiple files. We chould extract these into a shared enum in [src/icons.ts](https://github.com/confluentinc/vscode/blob/main/src/icons.ts) alongside `IconNames`, or split this into a more explicit `src/themeIcons.ts` and `src/themeColors.ts` since some colors may be used outside of the icon context.

- `"problemsErrorIcon.foreground"`
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/icons.ts#L48
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/environment.ts#L404
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/viewProviders/resources.ts#L367
- `"problemsWarningIcon.foreground"`
- from https://github.com/confluentinc/vscode/pull/3263: https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/consumerGroup.ts#L179
- `"notificationsErrorIcon.foreground"`
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/flinkStatement.ts#L341
- `"notificationsWarningIcon.foreground"`
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/flinkStatement.ts#L342
- `"notificationsInfoIcon.foreground"`
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/flinkStatement.ts#L343
- `"charts.green"`
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/flinkStatement.ts#L345
- `"charts.lines"`
- https://github.com/confluentinc/vscode/blob/71b27b3188bb45711f09e1a98ec6e66bb074cb59/src/models/flinkStatement.ts#L346