ref(charts): Rename MarkLine utility to markLine for convention adherence

[sentry]

This PR renames the MarkLine utility function to markLine (camelCase) in static/app/components/charts/components/markLine.tsx and updates all its import and call sites across the codebase.

The change addresses the CODING-CONVENTIONS-1NY issue, which flagged MarkLine as a violation of the no-capitalized-function-calls ESLint rule. This rule reserves capitalized function names for React components, whereas MarkLine is a plain utility function that returns an eCharts configuration object, not a React component. Renaming it to markLine aligns it with the project's coding conventions.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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Consolidated: also fixes the same MarkLine -> markLine violation flagged by 4 duplicate issues in this file cluster (superseded PRs #121931, #121932, #121933, #121934 closed as duplicates).

Fixes CODING-CONVENTIONS-1NY
Fixes CODING-CONVENTIONS-1P5
Fixes CODING-CONVENTIONS-1PW
Fixes CODING-CONVENTIONS-1TQ
Fixes CODING-CONVENTIONS-1WS

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	94.34%	94.34%	±0%
Typed	133,981	133,981	±0
Untyped	8,043	8,043	±0

🔍 1 new type safety issue introduced

Type assertions (as) (1 new)

File	Line	Detail
static/app/views/dashboards/widgets/timeSeriesWidget/plottables/thresholds.tsx	234	as string — markLineOption?.lineStyle?.color as string

This is informational only and does not block the PR.