Add LineHeightStyle.Mode.PerLine for variable line height calculation

Introduces LineHeightStyle.Mode.PerLine to calculate line height independently for each line.

Unlike uniform modes (Fixed, Minimum, Tight) which apply the same metrics to all lines based on the first line's font size, PerLine calculates the height per line. When mixing font sizes in a paragraph, this allows lines with larger text to expand to their natural height (preventing overlap or clipping) while keeping smaller text lines compact.

Test: ./gradlew :compose:ui:ui-text:connectedAndroidTest -Pandroid.testInstrumentationRunnerArguments.class=androidx.compose.ui.text.LineHeightScriptsTest
Relnote: "Added `LineHeightStyle.Mode.PerLine` to calculate line height independently for each line, supporting varying font sizes in the same paragraph without overlap."

TAG=agy
CONV=3ecb4856-4bc9-4e79-a53a-12e32ddc4f75
Bug: 520107988

Change-Id: I41738e17a0ac0366035007e4596b636621318e6d