Add BuildCheck for WriteLinesToFile without Overwrite

[iamhaseebn]

Fixes #11927

Context

WriteLinesToFile defaults Overwrite to false, so repeated incremental builds can append duplicate content when Overwrite is omitted. This adds the proposed opt-in style check without changing existing build behavior.

Changes Made

• Add opt-in built-in check BC0303 for WriteLinesToFile invocations that omit Overwrite.
• Preserve invocations with explicit Overwrite="true" or Overwrite="false" and match MSBuild names case-insensitively.
• Register the check and add resource strings and BuildCheck documentation.
• Add focused coverage for omitted, explicit, and case-insensitive task and parameter names.

Testing

• ./build.sh -v quiet - passed with zero warnings or errors.
• All 7 focused WriteLinesToFileBuildCheck tests - passed.
• Complete BuildCheck suite - 238 passed with zero failures and one expected Windows-only skip.
• Bootstrap sample build, help output, and end-to-end BC0303 diagnostic verification - passed.

Notes

The check is opt-in, so existing builds are unaffected unless it is explicitly enabled.

[ViktorHofer]

cc @baronfel in case you have an opinion here. @AR-May and I are OK with requiring to specify the Overwrite metadata under the assumption that this build check is not enabled by default.

[baronfel]

Love it, thanks for the contribution!