[select] Add multiple prop

[atomiks]

Preview: https://deploy-preview-1927--base-ui.netlify.app/react/components/select#multi-select

Closes #1956

[pkg-pr-new]

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npm i https://pkg.pr.new/@base-ui-components/react@1927

commit: 79b0631

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[mui-bot]

Bundle size report

Total Size Change:^{${\tiny{\color{red}▲}}$}+2.56KB^(+0.18%) - Total Gzip Change:^{${\tiny{\color{red}▲}}$}+894B^(+0.18%)
Files: 41 total (0 added, 0 removed, 2 changed)

@base-ui-components/react/select parsed:^{${\tiny{\color{red}▲}}$}+1.28KB^(+1.15%) gzip:^{${\tiny{\color{red}▲}}$}+442B^(+1.19%)
@base-ui-components/react parsed:^{${\tiny{\color{red}▲}}$}+1.28KB^(+0.42%) gzip:^{${\tiny{\color{red}▲}}$}+452B^(+0.49%)

Details of bundle changes

Generated by 🚫 dangerJS against 79b0631